Its hard to say.
Both of those tubes are "hard" as far as breakup is concerned, and very dependant on plate voltage... but, all things considered and given the same plate voltage, the KT88 will break up
marginally easier than the 6550 and is a bit smoother when it does. This only applies with plate voltages lower than about 600-650. after that, you are getting beyond the realm of maximum plate voltage of the 6550 and into the area of nominal plate voltage for a KT88.. after that.. youre popping 6550s like light bulbs and the KT88 is headroom city.
You will also find the KT88 to be more "open" sounding.. and to be of higher fidelity than the 6550. Highs will come through very clear and sparkling.. and you will notice bass that you didnt know you had. Its real experience.. but expect ANYTHING that goes through the preamp to be amplified very very well.. mistakes included.
If youre going to go with a KT88, i recommend the Svetlana or JJ. If your amp is going to get beat around alot, go for the Sovtek. The tone isnt as crisp, and fidelity isnt as good, but youre trading that for thicker glass and more robust construction.
If you are looking just for breakup, its going to be hard to get with a Sunn amp, as most Sunns are wired with high plate voltage and low tube saturation. You may want to have your amp converted to accept EL34 tubes. you will experience a drop in output power by about half, but you will have a smooth Marshall like output tube breakup.
caveat emptor if you do this conversion though..as it is costly.
