6550s vs kt88s for a mac 275 amp

i will be receiving a mac 275 mark 5 from a dealer in about 10 days. i expect the amp to be equipped with 4 kt88s.
while not my favorite output tube, it has its strengths.

i am wondering about purchasing 4 nos 6550s. i have no idea what to expect with this tube.

are there any reasonably priced which are more mellow in the treble, perhaps also a bit richer in the lower mids and slightly rolled in the treble ?

If the sound you're seeking in your preamp is what you want from your amps, the only 6550 I'd suggest would be NOS Tung-Sols. And other than that, I'd suggest getting NOS Gold Lion/Gold Monarch/GEC KT88s. As a general rule, the 6550s will have tighter bass, but a leaner midrange, than the KT88s; not sure you'd like that sound, from your other posts. The Svetlanas that Rich suggests are a current 6550 that appraoches the sound of the old Tung Sols, and may well be richer in the mids than current KT88s, I haven't really listened to the new ones.
i have an mc 275 commemorative and still have the tubes it came with. its incredibly musical even at the bottom. a second set also came with it, but i've never rolled them.
Roll the Chinese original Mcintosh 12ax7 and 12at7's with Mullards for warmth and smoothness if you prefer a rolled off softer top end.
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FWIW, regarding Reprince's comments and Tvads concurrence, in my experience the 6550s (SED's especially)is similar except I would describe it a little differently. I consider the 6550 to have a more linear sound with a softer high end than the KT88 which I have found to have, in comparison, a bit of bloom in the upper bass and some 'sparkle' in the upper mids and highs. The KT90 and the 6550 are somewhat similar except the KT90 is not short of high end energy. Which of these tubes sound best greatly depends on the amps they are used in, and, obviously, your system and tastes. I use both of these tube types in two different amps and neither tube was optimum when used in the other amp. Called system synergy!

One caveat for those considering 6550's who are seeking 'warmth'. You won't find it in 6550's (or KT90's). EL34's and 6l6GC's (if you can use them) are a good alternative for mid-range, and the 6l6GC has a softer upper-mid range highs. Quite a nice tube actually, but neither of thise have outstnding bass.

Mt T if you can do without the bass of a 6550 the 6L6 has tone you should love. The Mc probably will not use it though.