throwing another KT88 in the mix, i can highly recommend Valve Art Kt88's (or KT100's). they are very rugged tubes, with a high build quality, and my favorite sounding non-nos 6550A/KT88 tube. they can take alot of volts too.
they are actually made by Shuguang, and i never thought in a million years that i would recommend a Chinese tube, but i like them as much as my nos GE 6550A's, and more than the Genalex Gold Lion Reissues (which, IMO are very good), Winged C's, Sovteks, JJ's and EH etc.
i'm running 4 of them in my Sound City Bass 150, and my Sunn 2000S, and after A/B'ing them with the GE's, i actually like the Valve Art's better as they have a bigger, more extended and smoother low end, very even mids, and clearer, sweeter highs. the GE's have just a little more of that famous mid growl that they are known for, but i really don't miss it at all with the Valve Arts.