KT88 vs 6550 tubes

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 .

I probably have 50 or so 6550s or KT88s. Unless you want the NOS Tung Sol 6550s which are pricy, the Genelex Gold Lions are a pretty good choice. I've had the 275s and have been running MC60s for the last few years. The reissue Tung Sols have really good "shove" but are just slightly grainy. Sophia Electric very resolving but the bass isn't quite there. Good tubes though.
I do have a matched quad of NOS Tung Sol 6550s three hole grey plate which have never been unpackaged. I've never sold anything on Agon but would list them if you're interested. The bass, while not light, is just a bit tubey. Voices and instruments are by a good measure more real. Got them from Tube Depot I believe.
If you want the ultimate tube I just saw some NOS Black Plate Tung Sol on Tube Depot, the tube the original 275s were voiced with. $650 each (whew). But they are the best ever made, or so I've heard. If you are keeping the 275s for the long haul these may be of more value to you than may be apparent. Like the grey plates, but supposedly the bass is the best of all. I've been watching for 5 years and these are the first NOS Black Plates I've seen. 

@dentdog 

there are quite a few Tung Sol NOS black plates on eBay. Are they not the same? I mean there’s no way to fake those things based on what I’ve read. 

@dentdog actually there are also some original British Mullard KT88s. I’m not sure whether they are safe to buy because of my previous problem with the KT88s 

http://www.jacmusic.com/KT88/kt88.htm

Lots of info at jacmusic.