A couple notes on the plexi schematic. With some tubes, you may notice one of your power tubes glowing (usually happens to be on the tube closest to the preamp tubes). A couple tricks that really wont change your tone, yet will keep the tubes happier when the amp is cranked:: 
1) change the 10k 'tail' resistor in the phase inverter to 22k or even 33k to 'balance' the PI a bit more. 
2) change the 1k screen resistors on the output tubes (or if you don't have them at all add--) to 1k5 or 2k2. Tubes will be MUCH happier this way. 

Tonal tweaks you may like. -- change the bypass cap on the cathode of the 'normal' channel tube stage from the 220u or 330u to 10uf or lower -- 1 or 2 uf is preferred by a lot of guys-- this will keep thigns from farting out as much when you have both channels bridged. 
another tweak may be to use a smaller cap than the 0.05uf disc cap on the brite volume pot. Try a 500pf or even a 250pf cap -- you'll still get some brightness boost at lower settings, but it won't tear your head off with the high end. 

Finally, try a LOT of different preamp tubes!!! Mullard and Brimar 12ax7s are preferred by most (they also made them for other people-- so any preamp tube made in england/great britain would be one of those!). Telefunkens are also sought after. I personally like RCA's as well. As for power tubes, Mullards are considered to be the holy grail, but some also like the Siemens type. Svetlana in current production are nice, but some like the ElectroHarmonix EL34's as well. Tesla's are another choice for a raunchier tone. 
For even more fun, if you can get ahold of a set of 6ca7's (usa made), those are quite tasty! 
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1st stage cathode bypass caps are a great thing to play around with. I start with the stock values and then tweak-to-taste. On one amp I have a 100uF on the dark channel, on another amp I use a 4.7uF cap and get essentially the same results. 

Also, the 2nd gain stage cathode is bypassed by a .68 in the 1987 circuit. In my builds I've found that it just makes the amp too gainy for my tastes so I leave that cathode resistor unbypassed. 

Also, when it comes to volume pot bypass caps - I, too, find 500pF way too frickin' bright. My preference there right now is 100pF. That value was stock on Jtm 45's and organ model 50 watters (I think.) 
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