Getting to the end of my R8 mods. I made the changes is a bit of a different order. At some point I changed speakers and I started noticing a low frequency hum when the amp is turned up, or using the preamp input at any volume. After reading up on forums and the books you recommended, I realized I never moved the heater wire for the pre-amp. That got rid of a lot of the remaining noise and hum. Still has some hum though, but I have to practicaly put ear in woofer.
For me the best sonic improvement was aplitting the voltage stabilizer tube into separate channels, as for some reason my left and righ channels were out of phase, but after completing this mode, the channels were in phase on the oscilloscope and made a noticeable sounds stage improvement. Not sure if I broke something along the way, and fixed it, or if amp was always like that. Next best improvement was adjusting the negative feedback. I left the 33k resisors in, but bypassed them with 120pf silver mica caps, which resulted in slightly rounded off 1k square wave and pretty good up till 10k, severe rounding at 20k square wave, but 20khz sine wave is barely reduced in amplitude. It sounds now so good. The brightness that was fatiguing to me on this amp and my speakers are gone.