The answer is it could be modded to. But.......
The SYN50/50 power amp bias is set by a fixed resistor and not adjustable and is set very cold like Mesa does. That's why many who have used it found it somewhat lacking to other power amps and even power amps made by Fyrette. I am sure if it was modded to run the tubes a tad hotter it would sound much better.
The output transformers may not be optimally designed for EL34's on the primary impedance size, although it wouldn't hurt anything.
Also EL34's pull more heater current buy quite a bit, so the power supply may not be able to supply enough current anyway.
