You can get the HXDA in 50W or 30W and I'd say a half -power switch would be possible. For other stuff, here's a response from the PTC I got regarding other mods:
"The HXDA is a pretty well worked out design as all the switches and the dual volume controls really give it a tone of tonal options. We can do modes like adding an effects loop, reverb, or tremolo but in doing this would change the purity of the circuit.
We can add a parallel/series switch to take the HXDA topology to a higher gain topology like a JCM 800 but that requires a few changes to the original HXDA circuit."
I ended up just leaving the amp as is (30W 212 Combo) and couldn't be happier with the amp.
Hope this helps.