I've been recently experimenting with feeding the PWD2 some "better" juice. The stacking of 1 + 2 below has been transformational. The PWD2 stepped out 1 or 2 classes in SQ, mainly in: instrument separation, imaging, transparency, focus, details, micro-details. There is an even more natural and profound extension to anything from the pluck of a guitar chord to the human voice. The realism level is now breathtaking. It's 10 years+ since I last touched a guitar, but it's the first time listening through my rig that I get an eery feeling the guitar is in the room and there's no rig at all. I am also getting a feeling for the size and shape of the studio where the recording was made. (This is with Joe Bonamassa and my reference Stockfisch recordings).
1. Power regen. One of these cleaning-up the mains
http://www.powerinspired.com/ac-regenerators-c-98.html. For people in the US, I'm sure you are spoilt with choice. This is feeding no.2
2. Balanced PSU.
http://www.airlinktransformers.com/balanced_power_supply/conditioning_balanced_power_supply/ (I use a step down balanced PSU as I am the UK with an US version PWD2)
Also very worthwhile:
I still use the PUC2Lite USB->AES interface which gets rid of the slight confusion, darkness and mudiness of the stock PWD2 USB. The PUC2Lite provides a neutral tonal balance, improves the level of details and micro-details and expands the soundstage with a cleaner background. I've also added a SOTM txUSBexp card and that is at least as good (or better) than the PUC2Lite. The SOTM adds slam, better focus and PRAT and a natural extension at both ends of the spectrum.
My 2c. YMMV, FWIW etc.
The rest of the testing gear was:
SET tube amplifier (Amperex 6922 pinched waist, GEC U52/RCA 5Y3GT, Amperex EL84)
Custom 96dB front loaded horns
SR-009
PWD2 firmware 2.4.1 & 2.4.3