Not sure if anyone is going to be able to answer this one for sure. Here are some thoughts :

1. The ground pin is for safety, so that would also have to be a concern. I assume the rest of the house is like this as well, if not then think hard about getting some grounds in there.

2. The biggest hum problems come from ground loops (ie ground is connected but becomes the source of your problem) not absence of ground.

3. If you don't mind fishing a wire through the wall hooking a ground up is easy (you need 3 prong outlets of course)... I have heard about a few people who ran an external ground wire for one system and it seemed to be OK but ugly.

Bottom line -- most of my equipment has 2 prong power cords anyways so this wouldn't be a big deal for me. Does your equipment mostly have 3 prong ?

I would go ahead and run with the 2 prong outlets but be even more careful than normal about things like spilling liquids into the equipment. I don't think you're going to have a hum problem but if your equipment is designed for 3 prong there is a small safety risk (ie a small chance of something going wrong, but potentially serious if something does go wrong).

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8