KT88's are pretty much a drop in replacement for 6550's in most circuits, and your Sunn is no exception.
But that said, I'd check the bias even if I were replacing the 6550's with 6550's.
In fact, to me checking the bias is like changing the oil in your car, something that is part of routine maintenance whether or not you change output tubes. Things can drift, tubes can change as they age, ect., and maintaining proper bias is probably the easiest way to compensate for these changes and keep your amp running strong.
Of course, also like your car it comes down to practicality; I suppose the best thing for engine life would be to change your oil every morning before you left the house, but that isn't necessary or very practical. Ditto for checking your bias.
Also like only changing the oil in your car every 15-20K miles with the cheapest 30W available your car might be fine, just as your bias might be fine forever if you never check it.
I'd say it's your call; but it's easier to check/adjust your particular amp's bias than it is to change your car's oil. All you need is a cheap multimeter and a thin bladed screw driver.
Of course You can get much more elaborate about it, but if you are showing or can adjust it to around -55V with those simple tools you are likely hunky dory.
If you can't... Find a tech who can!
Good Luck!
