Upgrade tubes for Mac 275?

I have a McIntosh 275 Reissue with stock tubes, which I believe I would like to replace entirely with something better. The 275 is part of a biamp two-channel system, a Mac 2155 going to the low end of my Spendor 1/2s. Preamp is a C33; VPI Scout; Monolithic phono preamp; Earmax Pro & HD600s. This is a very small listening space (12x12), well-damped. A Mac guy my entire life, I love the Macs' classic sound with the Spendors.

Music includes folk, psychellic rock, classical, jazz, bluegrass and female vocals.

Recommendations are appreciated. I want to make the right choice, something that really brings out the best of the 275. I also want to be sure that I will be able to purchase sufficient spares for years to come.

Depends on when you bought your Mac as to which tubes were supplied by the factory.

Mac was buying Svetlana (St. Petersburg) 6550's until the name got bought out by New Sensor. After that, Mac took advantage of the discounts that were being offered and bought the new (Reflector) built Sovtek 6550 branded as Svetlana.

The original Mac 275 was fitted with (British) Genelex Gold Lion Kt 88's. That tube is so rare and difficult to find, it would be prohibitive to buy. There is a current model KT 88 built by several manufacturers in Russia and China, including Svetlana and Sovtek.

I have not conducted tests with a Mac to see what works best in that circuit, so you would have to experiment on your own or find a member with specific experience.