There is a typo in the "RC" BOM. Previously it listed the Bass pot with a value of 250kB and the Drive pot with a value of 50kB. These need to be reversed: Bass = 50kB, Drive = 250kB.
Someone was also nice enough to let me know that the stock IC is supposedly a JRC4557D. Info on this IC is a bit sketchy, but this customer did provide a link to a possible source for it: http://www.dalbani.co.uk/index.php
Thanks, Simon!
The pdf document for the Quasar has been updated to correct the typo.
Here is a direct link to purchase the 4557:
http://www.dalbani.co.uk/catalogue/product_details.php?id=27344
This has also been added to the project document.
One more correction: for the AC version, use a 1k for R8, not the 10k originally listed on the BOM. The original 10k/100n filter there is too deep of a roll-off for it to make any sense. I went back and looked at some of the various schematics and most of them seem to be incorrect in that area except for one which had R8 listed as 1k (done by JHS).
Alternatively, you could leave the 10k in there and replace that 100n with a 10n to get pretty much the same filter (this is what I would do).
Of course, if you like the way it sounds, leave it as is! The original combo will work well for bass, too.
Thanks to Mr. Simon W for pointing this out!