New Transformer on the Pole

I suspect that the likeliest explanation is simply that your line voltage changed somewhat as a result of the transformer change. Due to some combination of differences in their nominal turns ratios, differences in their turns ratios within whatever +/- tolerance they each have, and perhaps also differences in their internal resistance.

Another possible explanation is that the transformers perhaps have significantly different bandwidths, resulting in differences in the amplitude and spectral characteristics of noise that can be coupled through them from the high tension wires to your house wiring.

In any event, enjoy! Best regards,
-- Al

The newer transformers may have a lower inherent resistance/impedance. I don't know about old vs. new pole mounted, but for sure, pad mounted transformers tend to have much lower impedance, so much that the recommended AIR for service breakers may be higher if you have a pad mounted transformer than pole and are a short distance from the transformer.

It's very likely that the change in transformer was part of other changes in the utility's distribution circuit, including a possible change in the voltage of the distribution circuit itself, improvements made at the electric substation and even perhaps connection to another substation entirely. Electric distribution networks are dynamic systems and utilities frequently make changes to balance loads throughout the system. Any of these changes, including the new transformer, could conceivably affect your system.

almarg / cleeds -
curious about your feedback (no pun intended) here .... 

both of you have indicated that (paraphrasing) a change in line voltage could be the rationale as to the reason for a blacker background or perceived improvement. In my case I currently have virtually no "noise floor" (i.e. very black, etc.). Very happy in this regard! I am using a Panamax MR4300. That unit has a voltage level monitor.  My electricity service is provided by the "incorporated village" of Freeport and is known (rated) to be among the best overall electrical service in the US. 

My MR4300 consistently (when on or off and whether or not I'm listening to music) indicates the voltage is varying. It has measured as low as 116 and as high as 123. Regardless of the reading, I am unable to discern any audible difference. So my point in soliciting your feedback is to understand what degree of change you are speaking of, etc. Any other input is also appreciated.