There are some better sources for electrical issues like this around here (hullguy posted recently, and he's an electrician), but I've got some thoughts that might help you out.

1. That definitely sounds like a problem with your electrical service, not the fault of either the 950 or the 770. The 770 is capable of pulling a lot of current, although at regular volume levels it shouldn't present much if any more load than any other amp would. It is probably most severely affected by the dips in power, and may be the reason the system is cutting out on you. With activity on other circuits causing these dips, I wonder what size your house electrical service is. When I bought my house in 1999, it still had the original (circa 1954) 60amp service. In a modern home, with central air, refrigerator, clothes dryer, and all of the assorted electronics (microwave, one or more TV's, computers, home theater equipment, hair dryers, etc...), you can overload a 60 amp service without even blinking. While having a new dryer outlet installed that summer, I also replaced the 60amp service with a 200amp service, leaving me some capacity for future power to the detached garage. In the spring of 2002, I added a dedicated computer circuit, two circuits for the home theater (one of which is not even being used right now), and a couple of exterior GFI outlets on a separate circuit, and I still have spaces in the panel. The new service cost me around $1000, I believe. The dedicated circuits (which can only be installed if there is space in your main panel for an additional breaker) cost me less than $250 each (I had all four circuits added along with a closet light for $750 total).

2. Dimmers are notoriously bad about pushing noise back onto the electrical service in a building. We run into it at work (commercial HVAC and electrical design) fairly often, thanks to dimmed fluorescent and incandescent lighting and variable speed controllers on my fan and pump motors -- which are just really high tech electronic dimmers to vary motor speed. A dedicated circuit will help, but you may also need some sort of power conditioning on your home theater gear since this particular sort of noise can affect circuits throughout the house (for example, your lights should already be on a separate circuit from your receptacles, and you are clearly getting noise onto the receptacle circuits). A power conditioner from Panamax or even Monster could help a lot with the dimmer hum. Other sources of noise (such as vacuum cleaners and refrigerator compressors) can be largely eliminated just by getting the stereo on a separate circuit -- getting my fridge off the same circuit as my stereo was a big help.

3. The zone 2 portion of the 950 is purely analog, which means it is separated from the 950's digital bass management, so there shouldn't be any bass management taking place. It's possible that the change in space acoustics (as you noted) is playing a role. It's also possible that the HK had some sort of bass boost or tone control active that you got used to hearing.

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