What's causing hiss??

If you hear identical hiss in both channels, it's more likely capacitors on the amplifier side power supply. Checking/replacing preamp caps is also encouraged.
I would also replace/upgrade coupling caps that believe it or not also can go bad.
If the hiss is only to one channel, or heard substantially higher one channel vs. the other, than you can check/swap/replace tubes between both channels to ID the bad tube.

You should try a very quiet, low powered, tube amp with your Klipsch speakers. Maybe something from Art Audio.

I fully agree with Jperry. There are many to choose from at at all price levels.