how to convert this amplifier to an 15/18V supply?

Yes but why, using separate power supply for the pre amp when you could incorporate it in your amp. ,
Becuse of this i would end up in capacitor hell if i would try to incorporate an potentiometer into your design.

Treat R11 and R12 like a potentiometer
1729987543299.png
 
At the moment, the best-sounding amplifier I've ever built that can also be really loud - provided you have some experience with speakers and all the other stuff - is an SE with just one transistor, 12 V, 1 amp. The parts make the sound: 9 out of 10 electronic parts are not suitable for audio. It's not about "distortion" or "power" or "internal resistance" or anything else. None of that matters to the ear: it's an difference detector: noise, crash.

One record has been with me for > 30 years - typical 80s, 90s sound junk; too much playing around with the mixer -, and only with this SE do I claim to have resolved this record in terms of sound. And not because of "beautiful-sounding" distortions, but because of cleanliness and clarity: less noise and crash.

My advice is to go on with SE too.
By the way: a JLH 1969 can also be operated as a 3-stage SE, a 2-stage SE, a 1-stage SE and a 1-stage follower. An interpretation as well as PP is also true;-)
And first of all, remove its bootstrap c: untested, unheard, engineering error sounding junk;-)


 
In general: the better and faster it cools, the better. But: its mechanical resonances and sound also modulate the electrical signal. Mechanical damping may be necessary. And the mica disks, their material, can also be heard. Ergo: knocking and stroke test;-)
My little SE is currently running on an awful sounding thin aluminum part that was installed in a cheap China karaoke amp. But not forever;-)