Postscript: Some posters here recommend a Niagara 3000 or 5000 over the 1200. I have to +1. Garth specifically mentioned that the higher-end Niagaras are not just "bigger." They boast more sophisticated functionality and, unfortunately after writing so many words on the topic above, I’m thinking now that the 1200 may not have that ground-noise shorting circuitry that is a core benefit of the more expensive models. I don't want to spout incorrect tech information here, so this may be worth confirming online.
Also, one reason why power conditioning may have made such a huge difference in my system is that my house is completely solar powered. Rooftop PV inverters are ridiculously noisy, so my unfiltered AC was probably more compromised than most and the improvement I heard may have been more dramatic than most.
Still, I have no problem justifying the cost of the 5000 upgrade. E.g., pre-Niagara, an old $150 phonostage didn’t sound too different than a $750 upgrade. But post-Niagara, it was easy to hear the difference.