Gonk is correct. The power company sends 240V into your house across 2 phases. It is really closer to 208V. This is then split in the middle to make your ground / neutral giving you 120V on each phase. This is really closer to 110V.
This creation of a service ground / neutral at your electrical panel is why you see your circuits grounded on the same bar (sometimes).

This is also why you get much better noise performance (less possibility of ground hum)by running a dedicated circuit for your stereo. The neutral noise from other things in your house (lights, computers, fridge compressor,air conditioner) run back to the panel (and subsequently the ground there) instead of through you amplifier.

Long story short... You should not have any problems with your 990.