The KT-77 is a higher wattage version of the EL-34. In theory, using them would result in an EL-34 type sound, but with more headroom. I say in theory, because most of the OTS circuits get the majority of the tone from the pre amp section, and the power amp section is biased relatively cool and doesn't generally color the sound too much. There are exceptions, of course. For example, the Bluesmaster tends to get a lil more dirty/overdriven on the clean side when pushed hard due to the design of the PI allowing it to break down more, and the clean preamp has more overall signal output, particularly in the lows and low mids, and can push the output tubes a lil more.
If you don't want the typical EL-34 sound, and want to experiment with higher wattage/more headroom output tubes, I recommend the KT-88. They will sound more like a 6L6. A couple of folks here in the forum have used em with no probs, even on stock iron. Either way you go, be sure to use a bias calculator program, as both will require much higher bias current than a 6L6, EL-34, or a KT-66.

Hope this helps!
Gregg