6550 to KT88

I switched out my EH6550's to JJ KT88's yesterday in my Rogue M120's, quite a difference from the get go...much better bass, sound stage is wide/deeper and overall I like the way they reproduce the music better... so now I'm wondering if the KT90's will be any better? Anyone go from the 88's to the 90's and stay with the 90's? Thanks for your comments.
I have both the JJ KT88 and Ei KT90 tubes in my home.

While I prefer both to the 6550 tube, it depends on what your priorities are as to whether you should again make the jump.

The KT90 will take you back in the sonic direction of the 6550. More deep bass, less punchy midbass, more neutral sound, and a bit more top end extension. The JJ KT88 have a seductively punch midbass and sweet sound overall.

I wouldn't be surprised that if you very much liked the improvement the JJ KT88 have provided, you will not be so in love with the KT90.
A-gon member Jfacker recently purchased some EI KT90s from me as well as some Gold Lion KT88s, found different strengths in the two types, and is using both sets depending on the music involved. If he doesn't post here you might want to contact him. From my experience in Jadis electronics, the Gold Lions ruled the midrange and soundstaging, the KT90s were probably tighter in the bass and much leaner overall. I far preferred the Gold Lions, but not sure how the modern KT88s stack up to them sonically, as I have not tried the new ones.
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.
The KT90s sound more neutral, more like good solidstate. But they're still natural sounding, like most any tube. I like them. KT88s sound to tubby to me. Plus I like the little extra power of the KT90s. I'm staying with the KT90s.
I have a solution for your problem: biamp! Run your midrange-tweets with the GEC KT-88s and your woofer with the KT-90s.
Not a bad idea, JT! If I still had my Triangle Antals I'd consider it--they had separate mid and bass drivers. The Meadowlarks can be biamped, but they're two ways so the GECs would only handle the tweeters. But hey, if I get more three-way speakers maybe I'll give it a shot. I'll need another KT88-based power amp, though...hmmm, maybe I can sell one of my livers.

The JJ KT88's may surprise you, I guess until I get a set of the 90's I can't give you a good comparison. Buying 8 to find out I like the 88's better gets a bit expensive, although I don't know what I'm complaining about, I must have 6 sets of IC's I don't use. I need to put some of them up for sale here on Audiogon one of these days.
I have a rogue audio zues that came with the EH6550's. I was expecting the EHKT88. I was wondering if there is a big difference in sound staging and bass by going to the KT88, also which KT88 is the most well rounded? Has anyone tried the SED pearl cryo valve svkt88. The info I have read is that they are best modern production KT88. Kind of pricey for a new tube,$145.00 a pair. I have read good things about the jj KT88 and the winged 'c' KT88 from the original svetlana plant in russia. I have been thinking about trying the kt88 or KT90, but it sounds like the kt90 is going in the wrong direction. Are the electro harmonix kt88s better than the 6550s?
Be very very careful with the JJ KT88s. Their reliability was a disaster for me with them not lasting more than 6 or so months. Upon destruction, they launched a power resistor in the amp (ARC VT130) each time. I went back to 6550s, lost a little of the midrange warmth but gained a more neutral tonal balance and reliability to boot.
Thanks for the warning . I was considering the JJ's.
Jaq , Your concern over JJ is realistic but a maddening tradeoff because they do have the best tonal qualities of any current KT88.Both JJs I bought from a well known Canadian tube dealer went bad frying 2 different ASL amps--- expensive repairs both times.The word from a previous thread about this stated that KT88s are the most easily damaged tubes and do poorly in shipping. Try finding a vintage amp that uses 7591 power tubes. This is great tube sound.
I rolled the svetlana 6550 and ei's kt90 on a pair of jadis Ja-30 before i changed to the Jadis se300b's.
I loved the 6550 over the kt90 although the lost of punch on the bass region. The mids and highs never sounded more sweet and relaxed. Since I´m using 93db on the low octaves the less power on bass wasn´t an issue. The great "BUT 2 here is that mids are made with a vintage jbl driver and horn and are very sensitive to "Harshness". I think in a regular dome midrange-high driver the kt90 maybe the best way to go.
I have no major differences of opinions of the various tubes types, but as JaFox noted the JJ was (still is?) very unreliable. In fact of the new production tubes the only ones I have not experienced major failures with yet are the Svet 6550's (my primary tube of choice when combined in my amps with EI 6DJ8's). In fact one of my mono's is in the shop right now - I tried out a new set of KT88's - boom! - tube, resistor, and fuse went south. So did $150. I've got to learn how to solder.
I'm using 8 Ei KT90s in a pair of New Generations amps. They sound fine and have been 100% reliable for months.