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 ?

thanks