I went through a few octets of Sylvania's 6550s, until Upscale ran out and I couldn't find any more.
Uncle Kev recommended the Winged 'C' and those were a nice replacement, until the plant burned.
The GE 6550 variants were my next stop and I loved 'em, until they got too salty for my wallet.
I kept a good testing quad, that I've used for comparison against a few other brands and found the Russian Gold Lion KT88s to provide fairly close to the same presentation.
Roll on and happy listening!