The heaters for the 6550 are provided by the 372LX. This slightly over the winding rating. However I was told this was OK as I am not using the 5V winding. There is a small 15VA toroidal transformer which supplies the heaters for the driver stages. You could have separate HT transformer and heater. The HT is fused to each OPT separately with a diode from the HT to ground if it blows. This saved my 6550 when I made a wiring error.
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I think the 6550 are more linear and they meet the 100W rms requirements at the HT. There nothing to stop you using KT88's and increasing the HT a bit to get more output power. Mine are EH and I did find a faulty one, but that just happens. They were matched quads but not sure how much that matters.
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I have 450 V B+ and I'll use only one pair of tubes per channel, I'm also limited by the fact that Toroidy declares their OPTs at no more than 80 Wrms (even when custom built, even if they have some customer that got more power with no distortion).
I've not so much experience with big bottles (even in my guitar amps I've mainly used EL84 that I love and EL34, plus once KT88), but curves in UL push me towards 6550 too, even if I need to bring them a bit into AB2 (any reliability issue with that?).
On the other side, when I used KT88 in guitar amps, they seemed to have the same kind of mids as EL84 (that I really like in Hi-Fi as well).