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I've been reading up on the subject for a while, but a lot of my projects and ideas are in different stages of WIP, so take the following with the appropriate disclaimer.

For more complex audio projects, unfortunately the basic arduinos, teensys and the like don't do too well for signal processing, the main problem is that their analog-to-digital (ADC) and digital-to-audio (DAC) converters are not fast/precise enough to achieve reasonable fidelity. As such, they work better for control or generative purposes, ie. oscillators or lo-fi synths. This includes the ESPs, since even if they're faster the ADCs and DACs are still not good enough.

There are separate chips, audio codecs (combination of DAC and ADC) that have the features required for signal processing, which can be found in separate projects;

Teensy's have a separate Audio board ( https://www.pjrc.com/store/teensy3_audio.html ) and a guitar-oriented board in https://www.tindie.com/products/Blackaddr/arduino-teensy-guitar-audio-shield/ . For arduino there's a few music-oriented boards around, but nothing guitar-specific as such as far as I'm aware. Though you can use arduino environment to program the teensy as well, so arguably the previous ones are kind of the same if you don't look too closely 

Then expanding the horizon a bit, you can go on to raspberry pis etc with their own audio stuff, but at that point there's either different audio processing environments to consider or more system issues to consider since the RPIs are running a full keyboard, mouse and monitor operating system (if you wish).

I have a bunch of stuff like this that I'm slowly working on, unfortunately hindered by a lack of time/energy and reliable electronics skills...

There's a "hifi" mod to the Boss GE-7 that only requires a few parts and switching the op amps that's fairly straightforward. You should be able to find demos and instructions by googling if interested. Though I feel that the range of the slide pots change a bit, so you end up doing quite physically small adjustments for tweaks.

I like these https://www.aliexpress.com/item/10pcs-Thread-lock-panel-mount-1-4-6-35-mm-stereo-phone-Jack-Headphone-socket-3/32757986844.html

From the ones I've had before me, these have the smallest profile looking in on the jack itself, 13 x 11 mm (plus the soldering tabs). Only issue is that they stick out a bit too far for aesthetics when mounted, so I end up using a few washers on the inside so the jack end is pretty much flush with the nut.

Don't remember exactly how I came over the website about a year ago and thought it looked like an amazing deal, unfortunately none of the amps are available directly in Europe and the shipping + import costs would basically kill any savings to be made. I did spend enough time looking at it to determine that the amps and designs are essentially "white label" versions of some commercially available amps, based on internet forums. At least the 8, 10 and 12 watts seemed to be the same as the Laney Cub series of amps, as well as the similar Fame (Musicstore.de house brand) amps. Can't of course verify, so take it for what it's worth,  but I left with the impression that you could use the aforementioned commercial amps as a reference for how they would sound (or as the European alternatives, they're a bit more expensive but definitely still on the more affordable side).

Came across this earlier and thought I should share;

https://www.kickstarter.com/projects/vfepedals/vfe-pedals-klein-bottle-multipath-multiband-looper

The youtube video seems to demo the possibilities quite well https://www.youtube.com/watch?v=6xXUcbK6pwg

Might not be everyone's cup of tea, but I thought it seemed quite awesome  Full price seems to be on the level of a commercial pedal, but the DIY ones a bit more manageable though perhaps on the steep side for some. Ended up backing with the PCB+microcontrollers option.

Wrt SurfyBear price, I think I ended up paying 100€ for Trem + Reverb + Shipping, and with a reverb pan, adapter and enclosure you're probably looking at 150€-ish not including knobs (jacks and pots etc included in the kit). Drop 30-ish if you're not getting the tremolo at the same time, which is approximately 120 + some work, so something like a normal pedal but definitely less than Boss etc new and to my knowledge cheaper than commercial spring reverbs. Don't know how much the shipping and customs might cost depending on where you live, I think it's shipped from Italy so no problems in the EU though they charged ~20€ for it (but I got a reverb pan at the same time).

There's a bunch of spring reverb schematics around and articles on how to design your own if you google a bit. The Surfy one is based on a Fender 6G15 with the tubes replaced with JFETs, no other changes apart from what you'd need to do to accommodate that from what I've read so perhaps that might be an idea to recreate if you have the skills?

+1 for the SurfyBear, both the reverb and trem fits in a 1590 BB : http://www.madbeanpedals.com/forum/index.php?topic=26313.msg255050#msg255050

I'm still using it like that, but probably going to rehouse it in a surf green one with some nice graphics, as well as order a few more to at least have a separate tremolo and build a trem+reverb into a VHT Special 6 for some Princeton-vibes.

There's a marathon thread on surfguitar101.com on the SurfyBear reverb and at least a separate one on the tremolo for the interested

I have this PCB, but haven't started the build yet. There's a mod out there somewhere that adds a resistor to even out the volume difference in the no diode mode that you might be interested in.

Conceptually I'd assume it'd be a question of matching tempo to resistance for the speed pot, so you'd need some kind of microcontroller that counts bpm and you'd need to calibrate resistance to match the tap. The Taptation for PT2399 might lend some ideas, but you should probably also consider that the pedal doesn't really go all that slow with the default pot range (see the thread on the surfguitar101.com forum) and would need significantly higher resistance for much slower speeds. There is some tap tempo microcontroller code around, f.e. the sabrotone.com stuff built for ATTinys, and googling just now it seems he's selling some preprogrammed as well.

Probably doable, but probably not particularly easy.

I managed to cram both a SurfyBear FET Reverb ( loooong thread on surfguitar101.com https://surfguitar101.com/forums/topic/24295/ ) and the SurfyTrem into one Hammond 1590BB! The pedal has all the connections on the top (just enough space to fit) and has true bypass with an effect order switch, so I can run reverb into trem or the other way around. The build is from DIY kits by Surfy Industries ( http://www.surfyindustries.com/ ) , so most of the parts came included, only used the enclosure, knobs and smaller jacks from elsewhere.




The FET reverb is a recreation of the Fender 6G15 using FETs instead of the tubes, while the tremolo circuit is similarly "tube-based" from an amp circuit featuring normal and harmonic tremolo modes. Since the reverb circuit is in fact an amplifier, the FETs end up giving enough heat that they have to touch the enclosure for heat dissipation, so they're isolated with heat-conductive tape to not get grounded.

The build is tight, the PCBs themselves are slightly too large to fit nicely in this enclosure while still leaving space for knobs and all the connections, so I decided to try mounting them on top of each other with PCB mounts glued to the enclosure. The reverb board is below, while the trem is on top with a bit of tape to ensure nothing is touching. In the process I learned normal super glue doesn't really work with aluminum, so I ended up re-gluing a few times over (tightening screws was occasionally enough to twist and break the bond with normal superglue) until I tried a metal epoxy superglue. In the end I gave up on the mounting screw for the reverb board, since the mount wasn't tall enough to tighten against the board properly and I didn't think of using enough washers before it broke off for the fourth time... Wiring up the boards themselves was quite easy, but the bypass wiring is really fiddly because of all the jumpers and three 3PDTs in one box. Everything seems to be as compact as it can realistically be, at least with my patience level.

The knobs I received don't fit the pots in the kit so I'm using some placeholders for now, and I'm missing an on-off and rate indicator led (only place they'll fit is just beside the footswitches), so I'll have to go back in there, but it'll be interesting to see if the heat is affecting the build at all. I'm still monitoring the whole thing for heat buildup, the pedal is warm to touch but doesn't seem to be worryingly so, and after an hour everything seems to still be working fine.



A few more pics of the build, unfortunately I seem to have forgotten to take one with just the reverb and it's connections wired up: http://imgur.com/a/iXI0y

It would be cool to do something to the outside of the enclosure, but I've yet to acquire the appropriate skills and am working from my apartment, so I don't really know where to get started...