Electrical issue

I have an electrical issue/s that I could use some help with. It's a hum issue, but after exhaustive research here and on AA I still cannot figure it out. I have 4 dedicated lines for my stereo run approximately 75 feet with 10/2 romex all connected to Porter Ports. I own Von Schweikert DB-99's, which have a powered sub built in. I have tried lifting the grounds on all the equipment except one(and every conceivable variation,I think), I've tried plugging everything into a power strip and then that into just one outlet, I've even tried using other outlets(not dedicated). In the end, if I disconnect everything and just plug the speaker into the wall with no other connections I will get a hum thru the woofer only. I have even put a PS Audio P500 between the speaker and the outlet to see if it made any difference to the hum, but no luck. If I leave the speaker wires connected and turn on the main amp then I will hear hum thru all the speakers(tweeter/mid/woofer).

I called VS and spoke with Kevin and he said that the signal is split in the speaker before the sub amp which would suggest that the hum is coming from my electrical system rather than my equipment. IOW, if I heard hum only thru the woofer and not the other drivers when the main amp is connected then there could be a problem with the sub amp, but this is not the case.

My house is new and the electrical and grounding system seem to be in good shape. I have checked every outlet I could find with a polarity/ground checker and did not find any problems. My panel is grounded both to the water line and a spiral rod buried next to the house. According to the builder these are acceptable to code here in Austin, Tx.

The only thing that seems odd is that when my A/C turns on, and sometimes my TV, the lights will dim for a second. I had the builder double check all the connections and made sure the proper gauge of wire etc. was used to hook up the A/C, but nothing seems to be wrong. Is it perhaps the Transformer on the pole outside? It's definitely old as it looks a bit rusty up there(new house - old neighborhood).

The hum is not particularily loud, but i can here from my listening position 10 feet away when no music is playing. I appreciate any help you may have to offer, and I'll answer as many questions as you have because I'd really like to solve this.

Thanks Nerspellsner! I didn't understand clearly before what you meant. I will ask my electrician about it and hopefully get him to come out and give it a try.
Forgot to answer your questions...I have only one main panel. When we hooked up the transformer we used a subpanel in a temporary arrangement, but I wanted to try it before permanently installing it. The main breaker is in the panel, not the meter socket.
This is a great product for surge protection and power enhancement that installs at your main panel. This is whats inside of the old Seakay Linerover line conditioners. It will not help with the hum problem. http://www.usesmfg.com/
I ruled out the speakers today. Borrowed a friends and they hummed too. Next step is to find out if it's the city or the house.
RFI is not the prob. Does the hum have a bit of a buzzing component to it? Two types;60&120hz. 120 has this really anoying buzzing to it,60 is rather bassey of course. A mic and a o-scope, sure would be handy. Dumb question,is a TV,or some other electronic device near the speaker, and was it also near your test speaker?