Lots of folks like the Gold Lion KT88. I don't see how you could go wrong, but I've never listened to them. If they are available they do cost about $100 apiece. I've used and still do Tung sol 6550 in my Dialogue 2. I suspect from what I've heard folks say about the Gold Lions is that the Tung sols might be a bit darker in tone. They are selling for about $85 apiece now. Any darkness can be compensated for by selection of the 12ax7. I'm presently using EL 12ax7's and 12au7's with them but they are hard to find if you even can and somewhat expensive if you do. I've used Mullards and JJ's 12AX7. The former is on the bright side but does open up the Tung-sol's, the JJ's are an underrated tube I think, but they are mid-range centric and will tame a bright power tube (or system) some.
If your unit will accept smaller power tubes you should consider the 6L6GC. It is a very neutral tube (in general) and tonally is similar to a good 6550 or KT88. Linear, clear, and not thin or bright. Just less power. I've used SED 6l6gc's in my Dialogue One and just loved the clarity. But as with many of my tube stash they are no longer in production, ergo hard to find and costly if you do.
I tried the Mullard KT88's - they didn't appeal to my ear much. Bass was fulsome - I like tighter bass (on my system).
Presently new production tubes are hard to find and prices certainly have gone up!
BTW, FWIW, if you feel desperate to experiment, and are willing to give up a bit of bass power, you can play with a set of JJ E34Ls and some of their small tubes. The power tubes cost a bit over $25 each and believe it or not their small tubes cost as much or more. The power tubes can light up a dull system with out being aggressive. :-)
BTW, personally speaking, I would not invest my money or yours on "NOS" tubes for our Primaluna integrateds.
As this might really apply to you with your stuff it really is not much more than a WAG! :-)
