I work on Kustom Tube 12A combos from time to time and have a couple of these amps kicking around my house at any given moment.  If you replace the 4558 IC with something hi-fi like an OPA2134PA, these $100USD amps are positively charming and useful as a practice, recording or workbench amp.

Since nobody else has previously posted this schem, I figured I'd share it with y'all.  Good luck!