Do we still need 6550 or KT88 tubes?

KT120s are absolutely reliable not only based on my own experience of using them daily for nearly 2 years, but also based on my having rarely ever seen anybody say otherwise even when I posted a question regarding longevity. I think we "need" 6550s and KT88s merely for tone options and not because KT120s are "terrible", because clearly that's nonesense if only based on the KT120's overwhelming popularity due to performance and an amazing lack of publicized tone or reliability issues. This (A'gon) crowd is hardly shy about opinions, and from what I've seen the KT120 is nothing if not utterly accepted. I've never seen a specific new-ish power tube (and until some recent Chinese versions these were a "one option" tube from Tung Sol only) be so quickly adopted by manufacturers where I can reasonably assume they wouldn't risk an amp design on an unreliable or "terrible" tube. I've also talked to service technicians from more than one local high end audio shop (and a few amp designers), and it "seems" KT120s are a well designed, reliable, and great sounding tube that has earned its place as a huge success in audio. Also, most professional guitar players (and high end guitar amp designers) use either 6V6, EL84, or EL34 based designs as the trend has moved away from KT88s and 6550s, except in a few higher power demanding bass amp designs.

I believe the 6550 and KT88 tubes are still needed even with the Tung Sol KT-120 or now the KT-150 tubes. When I started out with my Jolida JD-502P I initially got the amp because it could handle the KT-120s and there weren't any other manufacturers of the KT-120s. It would keep the tube rolling to the minimum. Well the curiosity got to me and I tried 6550 and KT88 tubes and have found nice differences between each.

Like DonJr said the more the merrier.

I have asked my question because I would like to replace my current KT120 set of power tubes (because of the age).

I have Jadis amplifiers that can accept EL34, 6CA7, KT77, KT88, KT90, 6550, KT120 and probably KT150.

In the past I used 6550 in my amps and Gold Lion (reissue) KT88. I remember when I switched to KT120 about two years ago it was big WOW factor. Better dynamics, deeper bass, more lively and open sound. Since that I was not listened to 6550 or KT88 and I wonder if there is a good idea to go back to these old tubes - rather back to good Svetlana Winged C 6550 than KT88.
Maybe a better idea is to wait for KT150 tubes.

As for reliability I had no issue with KT120 at all, but had a lot of problems with KT88 Gold Lion including blowed cathode resistors.

I'm told the KT120 has a bigger bottle and won't fit in my ARC VT100, so I'll continue using 6550's unless I hear differently. Also use 6550's in my guitar amp.

I have to agree with Wolf Garcia, the KT120 has been widely used and approved by many. I've not read or heard reports of reliability issues. People seem to be very happy with them.