McIntosh MC275 VI preamp tubes

@ mrklas, I concur with @oldhvymec.  When I first got the MC-275 it sounded really good...much better than the parasound A23 I had.  However, it was a little bright and the bottom end sounded thin...again, still better than the Parasound.  I did seek out Brent Jessee, and bought a mix of Tung-Sol and Mullard's...I think (it's been 4 or 5 years), and it changed the sound, a little more well rounded overall, but still wasn't entirely happy.   I ended up settling on used RCA tubes that I got from an estate sale.  The best sounding so far.

@noromance , It could be the KT-88's, but the reason I'm leaning more toward the preamp tubes is because the Golden Lion KT-88 I purchased new.  I also have a Golden Lion new 12AX7 in the inverter spot.   As I mentioned, the 12AX/AT7 were used RCA's from an estate sale.  That's why I suspect those over the power tubes.  I will say the RCA tubes are just absolutely excellent with my system. 

I was emailing back and forth with Ron Cornelius from Mcintosh in 2016 when I first got the MC-275.  He mentioned that the tubes should last on average 5000 - 10000 hours unless driven hard for long periods of time (something that can easily be accomplished by myself as a preferred metalhead, hard rock, blues, then jazz fiend in that order).  He also mentioned that the Mark VI version has the color changing LED's when a tube goes bad, but it's only for a catastrophic failure.  It does not alert you when a tube is getting weak.  

My friend and I went in on a tube tester years ago....but I ended up moving only 13 hours away...so checking the tubes is a bit of an endeavor.

Years ago I heard that the Telefunken tubes would be the best if I could find them and afford them (at the time they were running about $300 - 400/tube).  I see that they are making new black diamond Telefunken tubes...has anyone tried them before? 

If push comes to shove, I'll pick up a batch of RCA tubes.  I am just beginning to worry about how much life they have left in them.  I just wonder if the Golden Lion KT-88's have been as good as they are, then how about their 12AX7 and 12AT7's are.  

The Black Diamond Telefunken tubes are are made by JJ in Slovakia:

"TELEFUNKEN vacuum tubes have been the benchmark of excellence in all audio applications, both production and reproduction, for many decades. This rich history continues with the introduction of new production tubes from TELEFUNKEN Elektroakustik, in partnership with JJ Tubes from the Carpathian Mountains of Cadca in Slovakia."

JJ tubes in general do not have a reputation for good sound, but the black diamonds may work for you, who knows? The one thing to be aware of is that new production tubes have nothing in common with the vintage tubes whose names they use. These new production tubes license the names from whoever owns the rights and that’s it.

Some of the classic tube makers sold their equipment to others when they shut down production. I think Telefunken sold some equipment to Tungsram and some Mullard equipment went to Japan, IIRC, but even then the tubes produced were not identical to the originals. They could be very good tubes though.

I tried the Gold Lions in my McIntosh C2500 preamp and didn’t care for them at all.  I went back to the McIntosh branded tubes which by the way cost me $15.00 each and they most certainly did not come in a wooden box with their name burned in on them. I wish they did though. 

+1 @tomcy6 .
I agree, don't expect these tubes to sound like NOS Tele's.
The risk in trying them is that JJ manufactures them, not known to be very durable tubes.