6550 and KT88 differences?

Did one have a clearer midrange?

In my Prologue 5 (W/Prologue 3) the SED KT88's have an emphasis in upper bass and a more forward midrange, smooth and extended highs. The SED 6550's had a more linear response with a slightly soft high end and a tight but not bloated bottome end. Tonally the 6550's sound more coolish in the mid range. I preferred the KT88's. The SED has a lot of color that can be attractive, but......

In a Cayin A100T (and 2 other tube amps I have) the two types have a similar sonic signature, but I much prefer the 6550. In it the KT88's bass was a bit much.

I'm guessing that the Dialogue might sound best with the KT88's, but much will depend on the brand of KT88 - they can sound quite different.

Any suggestions on a KT88 brand that's still smooth and extended yet maybe a tad less colored than the SED? Seems like I've heard mixed things about the EHs.

If I were to investigate new KT88's I'd be interested in the new Gold Lion reissues. Minimal input yet, but from what there has been they 'sound' like they might be excellent. Don't read too much negative into my assessment of the SED KT88. Of all the tubes I tried in my Prologue 5 (including EL34's and 6L6's)and other brands of 6550's and KT88's it was my favorite. All tubes are agruably colored, depending on your speakers and amp, not to mention sonic preferences. Also, don't overlook the impact you can benefit from in further refinement by changing the small tubes.

With respect to its design, the KT-88 is the British variant of the 6550.

In use, they have proven less reliable than the 6550, particularly back in the 80's when their were some especially poor quality Chinese KT-88's that were more roman candle than output tube. VAC sells very good KT-88's.

It depends upon the use to which the tube is put and the production run, but in my experience, KT-88's tend to be a bit richer sounding than 6550's, while 6550's may be more accurate and linear.