When it comes to amp sound, particularly tube amps, all that matters is what sounds good to the player. That said, here's my 2 cents worth... I think 6550's or KT88's will give you a bit more headroom and hold together deeper before breaking up. They both can pass more current than a 6L6. Most 6550's are a little more musical and less sterile, generally have a more narrow clean frequenct response range than most KT88's but will distort quicker. DEPENDING ON THE PARTICULAR TUBES! Not a drop in replacement, but any competent tech can do it easily. It really depends on what you like. If you want real loud and clean "chimey", but still tasty tone for guitar get a Reeves or maybe a big Hiwatt.
I think most people would agree that 12AX7's are generally the hot ticket for pre amp circuits.
