The transformers would never be more then a band aid for the ES-1 since the circuit is more then capable of running SE since that's how Stax designed it. I had these same issues with my mini ES-1 so adjusting the resistor values and using the right tubes should clear it up. I still need to buy a quad of some nice 6SN7's to try in the amp and if that clears it up, drop some loctal sockets in there for 7n7's.
Big, no... gigantic, yes. Building a new enclosure for the SR-Omega drivers would be a really cool project though even if it would cost a fortune to get it right.