I would NOT cut up an old PA unless it was total junk. I use a Fisher
KX-200 with 7591A outs. I doubt you could improve it. You also
cant buy anything new that will touch it for under a grand at least.
If you want to build for fun think about using odd ball tubes that
can be MUCH cheaper. See the prices of 7591a, 6CA7, 6L6 etc.
Dont worry about brute wattage. With good SPKRS 5-10 watts / channel
give room filling sound. Building tube amps is a pretty big hobby. I bet
there are tons of sites for it. Just start small & if you run into anal orifices
just ignore them. They are out there.
73 Zeno
LFOD !