Jfacker reporting here...the differences between KT88s and the Ei KT90s are pretty much exactly as Rcprince describes. The first upgrade I made to my ASL integrated was replacing the stock chinese KT88s with Electro Harmonix. Very big improvement--tighter bass, lower perceived noise floor, more extended treble. Generally good dimensionality too in terms of width, depth and height. I tried some SED Svetlanas from Upscale Audio and found them too syrupy and slow for my tastes, with weaker bass, indistinct imaging and a veiled sound in comparison with the EH. Then Rcprince turned me onto the Gold Lions and the Ei KT90s. The Gold Lions are vintage tubes and produce the widest and deepest soundstage I've ever heard. The imaging and holographic effects I get with these tubes in my system (Meadowlark HR Kestrels, modded Sony SACD/DVD player) is downright freaky. With the KT90s, on the other hand, a lot of those wild soundstage effects are reined in, at least in terms of depth. BUT you get wickedly tight, deep bass like I've never heard from any other tube and the highs are very sweet (I actually prefer them to the highs of the EH tubes, which can be very slightly fatiguing to my ears). With the KT90s in my amp it's less "tubey" sounding, it sounds more like a very, very good SS amp--very clean, utterly grain-free, powerful but refined, etc.
Haven't tried 6550s, although a friend recommended vintage Tung-Sols, and I haven't tried the JJ KT88s either. Would like to check those out.
Of course, the net effect of all these fluctuations in my system has been to make me yearn for something that will let me have it all: the gorgeous midrange and dimensionality of the better KT88s and the killer bass of the KT90s. The variables represented by the different sounds of different tubes have been enough to make me want to hang up the whole thing and go back to solid state, but I have to say, on the whole, regardless of what's in the amp, I get more enjoyment out of my MUSIC with the ASL integrated than I ever did with any SS equipment including Classe, Audio Refinement and Nakamichi STASIS stuff. And isn't enjoying the MUSIC what it's all about? Tubes, for me, make listening to my CDs a serious activity--it's like they take me from a spectator to a participant.
Next stop? Vinyl. I have a large record collection but I've never had an even halfway respectable turntable and at the moment the collection is in storage. If what I've been lead to believe is true, the combination of high-end analog and tubes may cause me to transcend my earthly form and I'll have to beam future contributions to this forum directly from the astral plane.