Are KT-88 a good substitute for 6550c?

According to the "tube Substitution Guidebook" by HA Middleton, 6550 is an excellent sub for a KT-88. If you are worried about any harm/damage, don't worry. Switching these will only change the sound, not the electronics.

I have my shares on KT-88s and 6550s on my SF Power 2 and my previous ARCVT100.
From my experimentation and my music taste, I liked Svetlana KT88s and JJ Tesla KT88s the best(out of current production tubes).

Svetlana gives little warmer and weighted presentation of the music where as JJ Tesla is more detailed.

I've compared Svetlana KT88's and Svetlana 6550c's in my Rogue M120 Magnum amps and preferred the Kt88's easily. My experience between the two types was excactly as Rcprince described above. The Kt88's richer and more analogue sounding, the 6550's brighter and less refined.

dudeaudio

You might take a look at audioasylum.com and the tubes forum. A search on 6550C or KT88 will turn up a great number of opinions on just that comparison. I found over 200 hits on just a quick search.

Zaikesman had a thread on this that you might look for; unfortunately, the KT88s I lent him did not bias properly in his amp and he hasn't been able to try them, but I now think they may have been worn out from too many years in my Jadis, which tend to eat up tubes. I may lend him some virtually unused Gold Lions to try. My experience with Jadis, rather than VTL, amplification is that the KT88s have most of the bass power of the 6550s but a much richer and harmonically correct midrange--the 6550s sound thinner and coarser, less refined in that area in comparison. Can't tell if this will hold true in the VTL circuit, nor can I vouch for the Electro Harmonix tubes (the Gold Lions are to die for, though, IMHO), but it does seem consistent with what you were told.