how to convert sunfire tube pre to 220V operation?

I am not familiar with your unit, but if I were in your shoes, I would unplug the unit from the powerline, wait about 15 minutes for the powercondensors to discharge and open it up to see, if there is a simple switch for changing voltages. If there isn't, possibly the transformer has more than one possibilty for the powerleads to be soldered on to it and if you are lucky, they have markings for say 100,115,230 VAC. Then changing the leads is a cinch, but you must take care not to mix up the positive with the neutral. If you are not sure, see a technician. If they have no markings, bring the unit to a technician anayway or use a stepdown-transformer with it in the first place. The difference between the 50/60 cycles should not matter in your case.

Bring the unit to your local tech and 99% he will be able to switch wires for 220Voperation for much less than dealers would charge to you.
Vast majority of power trannies in components are already pre-designed for 220V and it's a matter of few minutes to re-connect. Some companies charge for that $200 or even $300...