KT-77 vs. KT-66

KT66 will definitely sound different than KT88.  KT88 is Europe’s answer to 6550.  My Inspire will run both.  I love the St Petersburg Svetlana 6550 in my Inspire, also have GL KT88 which is slightly different than the 6550, hard to put my finger (or ear) on difference.  Also have Shuggy Nature series black bottle which is a very nice sounding tube, closer to GL, further from 6550.  I really like all three.

What’s really fun is swapping the driver tube between 6SL7 (higher gain, more top end) and 6SN7 (lower gain, less top end detail, smoother and touch darker).  The Inspire runs both.  Great combo is the 6550 or Nature bb w a 1945 JAN RCA grey bottle 6SL7, get warm dark bass and mids, with the 6SL7 providing more detail and sparkle on top. 
 

obsessed and addicted 

What do the KT88s sound like?

@jimmy2615 The GL KT-88’s in my system have "slam" and bass response; lower mids have punch. Mids and highs are a bit harsh; this can be tamed a bit with changes in my preamp 6SN7 tube.

Curious how others might answer your question.

Thanks.  I thought I’d also add, after noting some of the comments about 6L6s, I have some NOS RCA blackplate 6L6GCs.  These sound like JJSS was describing above.  The KT66s are a lot more dynamic and not as warm in my system.

@jimmy2615 I have a Primaluna Dialogue Premium integrated and have rolled tubes a bit, including KT88’s and 6550’s. I, always, come back to KT77’s. To me, I find KT88’s (Tung Sol & GL) and 6550’s (EH) have more bass, but loose significant detail. KT77’s have it all. I was not impressed with Siemens el34, either. I would like to try some Amperex el34’s double-D getters, but need another matched pair. Also, I would like to hear how vintage 6L6’s sound in a Primaluna. A friend swears by them in his guitar amp.  Though, not the same equipment. 

I've been using my Dennis Had SEP tube amp again lately (I switch between it and a Pass XA-25) which can utilize a really wide variety of "KT" type tubes and I really like KT77s (Gold Lion) in the thing. A fave for sure. I discovered a  forgotten  pair of GL KT66s I bought just before the Russian tube world imploded and recently stuck 'em in the amp...it seems they need a little while to break-in...they're great sounding! Really hard to say if they're better than 77s or to describe the difference, but hey...try 'em! The Gold Lions are great looking tubes by the way.