Because I have a lot of work to do, and this is a very complex subject (and I have no idea how much electronics education you have), I'd prefer not to go into a lot of detail about this.
I can give you a few pointers: 1) modern tubes have more distortion than NOS (in general). So more feedback is helpful in making current tubes sound their best. It's usually not a big change. 2) More plate current produces better linearity. However, one then has to run lower voltage to keep dissipation in the rated area. This is necessary anyway because modern tubes usually don't have very hard vacuum or the best materials. 3) esp. in guitar amps, adjusting or adding EQ can SOMEWHAT compensate for the poorer cathode materials in modern tubes. This is a MASSIVE area of study, one that nobody will ever write a textbook about because guitar amp mfrs. are notoriously secretive about their "magic formulae". Competition in that business is VERY cutthroat.

>Where can we read up on your little portable amp?

Do you mean the Metasonix TM-4? Nowhere that I know of. No publication has ever reviewed it. Sales have been very low. If this keeps up I'm tempted to either change it completely or drop it.
If you want to try it, I might be willing to discuss a temporary loan of the last one left in stock. You could always review it for some website, like enjoythemusic.com or 6moons.com. Email me. (synth@metasonix.com)