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Unregulated supply on the output of a push-pull emitter follower within a feedback loop does not normally contribute much to the sound - the PSRR (power supply rejection ratio) usually is pretty good. Pretty much all power amps work that way - it's not necessarily inherently bad. It can depend on the PC board layout, etc. If improving regulation to that output stage improves the sound, then there are inherent design problems with it. Perhaps there are, and maybe it'd help. But improving the supply before regulation might be a better first step.
Upping the main filter caps to reduce the ripple some wouldn't be a bad thing. The designer(s) probably did not want the output stage doing two things if it were connected to the regulated supply. First is overheating the main +/- 15V regs, the other is polluting the +/- 15V rail if the output is heavily loaded. I wouldn't necessarily move the outputs to the regulated supply - or if I was to dabble in regulation, I might regulate them to +/- 18V or something like that. If you up the output capacitors on the voltage regulators, make sure that you understand the powerup and powerdown ramp sequence. You can easily blow up those regulators if the output stays higher than the input. Conveniently, when these regs blow, they usually short input to output, so you can take out most of the IC's if you're not careful. Some designers put an inverted diode (from output of a 78xx/79xx reg) back to the input so if the output voltage is higher than the input (happens during turn-off), the reg doesn't poppencorken. I've blown up a few regs (and boards) doing this, and so I'm much more careful now.
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