No ground, should I worry?

The concern manufacturers have about grounding is for liability and safety. They can say I told you so if you fry your rear. If it sounds good then you are not effecting your system or doing any harm. Just don't touch it while in your shower. I had to cut the ground pin on my ARC LS2B to stop a terrible hum ground problem and the preamp worked great for the 4 years I owned it.

Check the buildings basement or an unfinished wall/ceiling to see if your wiring appears to run inside a pipe (conduit)or spiraled metal jacket. You may have easy access to ground using an inexpensive screw mount 3 prong to 2 prong adaptor. Using an inexpensive outlet testor and the mounted adaptor, check to see if you get two amber lights and no red light.

I own some Theta gear myself. They're pretty adamant about grounding their product. If you have any warranty work done in the future I think I'd leave out the ungrounded part when talking to them.

Where there is a will there is a way. I’ve been an Electrician for twenty years and I can pretty much guaranty you can get ground to that outlet one way or another.

While you’re at it you mine as well run a dedicated circuit or two.

Good luck :~)

PS- This is only my personal opinion. Normally I would agree with Bob but grounding is something I take very serious in my line of work.

re: Grounding: If it ain't broke then don't fix it.
BTW fuses sound better than breakers: try to have an old ceramic fuse in the circuit sourcing your audio rig vs. a glass fuse which doesn't so sound good. The newer plastic body fuses seem to be OK as well.