Right Joe the legend has been and gone, don't go near 6550's or KT88's, either stick to the EL34's or go for the JJ KT77's as suggested by Spendorman, but if you go for KT77's you need to reset the bias to 2 volts as specified on the spec plate on the amp, all this is asuming that the amp hasn't been got at before with things being changed from original.
I hope this helps