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A chip change won't do much. Instead, remove R10 and replace it with a 1M pot wired as a variable resistor. This will allow you to reduce the gain between the two tube stages. If you don't want to permanently leave in the pot, find a position on the pot that gives you the best compromise in conjunction with the Gain control, remove the pot and measure the resistance at that position, then substitute the closest value resistor back into R10.

Starved plate tube designs, like this one, are inherently more distorted than full voltage designs for a given input signal level.

I found a layout for the Red Box II here: http://rapidriotboxes.blogspot.com/2009/02/di-boxh-red-box-mark-ii.html
Schematic is here: https://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=31055

As you can see, it's just a passive treble peaking filter followed by two Sallen-Key low-pass filters. Not terribly exotic, and not as complex as the Condor. I've got a Red Box III (passive version) and it's useable in a pinch, but nowhere near as realistic as the newer digital convolution boxes that can load cabinet IRs.

If you're looking for something on the cheap, just get a Behringer ULTRA-G GI100 DI box with cab sim and call it a day.

Carl Martin Rock Bug
ADA GCS-2 or -3
Hughes and Kettner Red Box
Logidy EPSi with C firmware (permits loading different IRs)
Two Notes Torpedo CAB (also loads different IRs)

Sounds like the bottom end of the pot (toward the CCW direction) is no longer connected to ground. Make sure you didn't lift the pad for that lug/pin (assuming board mounted pots). You can always tack a wire from that lug to a known ground point, like the sleeve connection of the output jack.

You can dink around with Bax tone stack values using the Duncan Amps Tone Stack Calculator. Use the James tab (which is the passive version of Bax circuit).

Yes, R23 should be installed. It's not perfectly clear from the BOM, but R23 looks to be grouped with R10..R16 and should be 10k.

R2 and R9 aren't necessary, but you'll need to install R24 for the bass pot to work. The BOM calls for 220k in that position. C18 can be used to roll off some fizziness. If you install some socket pins, you can try different values in that position starting with 1n, or just leave it out if it sounds OK as is.

From the Self site:

"The "Superbal" configuration (figure 12) gives much better input symmetry than the standard differential amplifier. The differential input impedance is exactly 10k via both hot and cold inputs. Common mode input impedance is 20k for both inputs."

I figured if one were to use a dual opamp, they might as well use the other amp for better performance.

Yes, that will work fine. You can also do a balanced line input with a single or dual opamp and a couple of resistors on perfboard if you want to keep it all in chips.

From the Douglas Self site:

Ted Weber sells a little Balun transformer (balanced/unbalanced) for $4 that can be board mounted. Their hookup diagram shows its use as a line out, but you could use it for both input and output on your effect. Low-end freq response isn't that great, but for vocals it should be fine.

I got one from Amazon, and while the reverb sounded ok, the noise level was too high, even in bypass! Back it went. I ended up finding a TC Hall of Fame for about $100 and am perfectly content with it.

Jeanne at BNP Lasers does great work, but really only takes CorelDraw files. If you've got CorelDraw, you're golden.
Sandy at PrecisionDesign lasers also does great work, and takes more input drawing formats.

Pretty sure Joe at amplates just does the design and sources the actual mfg to BNP. Or at least he used to.

Also, pedals can be used on tracks, but there may be an issue with signal levels, particularly when used as insert effects. Inserts are usually at line levels, whereas most stomp boxes are designed for instrument level use and can be readily overdriven at line levels. You might want to look at an Ebtech Line-Level Shifter for matching levels between the insert and the pedal.

Back when I was part owner of a small recording studio, my partner and I built several pieces (what actually got me started in DIY). Part of that list includes:

-- LA2A compressors using the original UREI T4B modules and UTC transformers.
-- Pultec EQP-1A eq (Gyraf variant)
-- SSL talkback mix buss compressor
-- RCA BC-2B two channel mic pre using original RCA modules
-- Altec 1566A two channel mic pre clone
-- Langevin AM16 two channel mic pre using original Langevin modules

Then I started building guitar amps, after that pedals (thank you Brian!)

We've got one of the Owons at work, and it does have a nice display and plenty of features, but after a year the encoders are already getting a bit squirrely. Maybe for light shop use it would hold up better.