Need Help: What In My Home Electric System Killed the Sound?

You can get close to a dedicated line by tracing the "audio used" outlets back to the breaker box, and shutting off everything on the same electrical line. Turn off all florescent lights, the air conditioning/heating, and rheostat controlled lighting.

Try that, and see if the problem goes away

Clarification:  Go to the breaker box, and switch on and off the circuit breakers, and observe what shuts off and on, to trace the individual lines.

Thanks for all the suggestions and comments.  Alas, we are still in the process of elimination.  Since my OP, the electrician has retraced every step and assured proper connections and grounds, etc.  Importantly, he has checked the voltage with a fancy device and reports that voltage is exactly 115 with only tiny (non-material) variations.  Extension cord running off different circuits produced no improvement.  So, we have ruled out house or Maine Central Power as the cause.  (Note, however, that this home is at the foot of a mountain, gets hit hard by storms, and the locals tell me three weeks ago the thunder and lighting in the area were epic.)  Next, in the system, I replaced the CDP source, but no change.  As I see it, that leaves two candidates: (i) McIntosh C2500 pre-amp (correction from the OP, its a C2500 not C52 -- I had an early senior moment) or (ii) MC452.  It being the weekend, I wasn't able to talk to McIntosh to get guidance on which might be the target.  So I rolled the dice and am shipping the C2500 in for a check up (couldn't deal with packing up the 110 lb beast).  Fingers crossed its the pre-amp, and can be a quick fix; I am having the wife's family up for the Fourth and desperately need the rig in full throat to drown out the sister in law's non-stop mouth.

I had a very similar problem before I upgraded the homes wiring. My electric Bass rig would have low volume or cut out all together. It turned out to be an old receptacle in the garage which was used next to an old work bench that was simply trashed. Once replaced the rest of that circuit worked fine. 

Hi Whitecap, the first thing I think I would do in your situation is to simply sit tight and wait...at least a month to 6 weeks or so. Home electrical systems can be thought of as a ’single large circuit’ that, with your own typical use patterns, has reached (more or less) a state of homeostasis. Anytime you make a change to the circuit (in this case the wiring) you run a risk of disturbing that homeostasis until enough time goes by for it to recover. If 6 weeks go by and there is Z-e-r-o change, then I’d be looking for a problem with the electrical system and not with my gear that I know happened to be doing quite alright before the alteration. In the case of a continuance of the sound problem, I believe I’d start with asking the electrician if he is qualified to be able to give you a reliable verdict on the state of your home’s ground (assuming it was originally designed with a ground...your original duplexes would all show 3-prongs, not 2). If he’s not confident he can be very accurate about the test results, perhaps he may know of someone who is (the true, correct method for checking ground properly can be tricky for the novice). Also, the effectiveness of the grounding for homes in many parts of the country tends to be at or close to nil after about a decade or so. Your electrician should know all or most of how to restore good grounding, the only tricky part may be accurately measuring it. While you’re at it, ask if your electrician if he actually disturbed any part of your home’s ground system in the course of his work...I know sometimes old connections that were broken can have much higher resistance when rejoined if not cleaned properly.

p.s. sorry to hear about your sister-in-law...my advice is to just get some old earbuds to your phone and just make hand gestures like you can’t hear her... :>)