I used to have these amps. Never ran 6550s in them only GL KT88s. Stay away from the Chinese KT88s unless you like tube fireworks. They will not hold up to the B+ voltages on the 450. If you want a warmer sound check out the Mullard re-issue KT88s. If your preamp is fully differential, use the balanced inputs on the 450. It will be more dynamic and greater imaging.
