I've never seen output valve heaters in any serious amps left floating andy. If the heater winding isnt centre tapped just use a couple of 47ohm resistors to make a CT reference to earth. I wouldnt mess about with humbuckers when using DC heaters, why? But a neat trick when using something of the ilk of an ECC82 with 12.6V heaters is to take your reference point from pin9 which is the heater CT, I've done it and it works. I am assuming here that the DC supply is floating and the negative side isnt tied down to earth?
finally, was there any particular reason for using separate heater supplies for the cathode followers and the input /phase splitters? Again I have no qualms about using an elevated supply on the valves at lower cathode potentials, my KT88 amp runs elevated heaters for the pahse splitter and first stage which is an ECC83 with a cathode voltage of 1.5V above ground. Never had a problem.
Hope thats of help and still most impressed with your work.
Andy