KT88 vs 6550 tubes

Forgive me but, that was unwise. You need to ALWAYS adjust bias in a fixed bias circuit if you change tubes, let alone tube types. 6550 and KT88s are NOT swappable without doing this.

Did the KT88 plates that blew the fuse turn cherry red by any chance before it did?

Well, I don’t know man. The MC275 doesn’t even have a bias adjustment knob and manual says I can use both. No, no cherry red, just the golden glow like the color of a light bulb. 

I’ve had similar problems . The best buy , unless you want to spend 2K $ on NOS (which may not be same) Sylvania 6550’s is to to buy a matched quad from "the tubestore.com " This Canadian firm is honest
and does a good job .I got a Quad of KT 88 from them few months back to replace 6550’s only to see the sound difference . In My AES-25 Super Amp the 88’s are a bit clearer which I like .

The 6550’s ran for 4 years and are still fine . tubestore calls these quads "Preferred" , only few bucks more .