KT88's for me where more sterile and harsh sounding than any other tube. They really lack a lot of mid's to them. I've done a KT88 vs 6550 blind recording test, and sent it out to a bunch of friends that are tone geeks like me, and not a one person picked the KT88 tone over the 6550 tone. Most said, that the low end of both are very tight and punchy, but the 6550 tone was a lot more smooth and buttery and punchy in the mids. The highs were also more chimey and sparkly without sounding brittle and grainy like a KT88.
As for the VHT 2/50/2 vs 2/90/2, I have owned both. The VHT 2/50/2 I have ran EL34, 6L6's, and 6550s in it. With 6550's you will have to change the resistor value going into the bias pots, which I had an amp tech do at the time. I compared the 2/90/2 w/ 6550's and the VHT 2/90/2 had more depth, clarity, punch, and sparkle to it. The 2/90/2 was developed on the circuit of the VHT 2150 power amp, which is held by many as the "be all, end all" power amp. I've done side by side tests of the 2/90/2 and 2150, and the 2150 does win out on tone ever so slightly, but on weight, and being a 4-space rack amp, the 2/90/2 was the better route for me. Tonally if the 2150 is a 10, the 2/90/2 was a 9.5. The 2150 had a bit more depth to the tone. The 2/50/2 is the way to go if you are a EL34 guy. That amp rocks w/ EL34s. If you want a higher headroom power amp, get a VHT 2/90/2 w/ Svetlana 6550s, and you'll be in tonal heaven. Two of my favorite guitar players use amps w/ 6550s. Stevie Ray Vaughan and Zakk Wylde! All of SRV's Dumble amps had 6550s in them. Beautiful clean tones come from it...as well as balls to the wall heavy tones. By far my favorite power tube!
Eric
