Kt88 or 6550 tube for McIntosh MC275

Thanks for the reply.  Yes, it is auto bias.  

Is that an early MC275, or the later model? I notice a much greater difference in valves in the early model.

The preference to KT88 is because GREAT 6550s can break the bank.. 500-1000. a pair More than NOS KT88 GL

6550 GE TungSol, Old stock. That is a different sound than a KT88s

I like both, I have both Solvex and Golden Lyon (not my favorite). Good valves.

I have a few sets of 6550 GEs. I also found a Tube Tec valve that was pretty darn good.

I like the KT88, bottom to top, if the valve is going to be doing bass duty, too.

If it’s in a mid/highs, I use a 6550 GE. It’s just tough to beat that sound. It’s a little slim in the BASS department, but VERY smooth mids, and wonderful highs.
If you’re a vintage Klipsch, horn kind of guy.. They just do a great job with Macs, MC60s, and older 275s, sporting that valve, and Tellies/RCA for the signal valves.

Just so you know PSVANE makes a heck of a valve, over the last few years..Power and signal.. ShuGuang 6550s, are great valves also and without breaking the bank..

Very important to get matched valves with the old Macs, Yes it matters. Mac is different, not like anyone else... A Mac will show you every flaw in your system. Mismatch valves cause an early breakup, on the top especially. MATCH the valves...

Enjoy the sound..

Regards

Any idea how good or bad is the stock tube compared to the, say, Genalex Fold Lion reissue?

It is a MC275 MK IV.  
@oldhvymec when you say matched, do you mean two matched pairs or a matched quad?  I read somewhere that since it is a push pull and auto bias, two matched pairs, one for each channel may be fine.  

I have a mkIV as well and found greater upgrade to sound by rolling in NOS for the small signal tubes. 

I did also replace power tubes with GL kt88s which made what I felt was a smaller improvement compared to the small signal tubes.