I stumbled across this thread while looking for more information on Jplay... and now that I've tried it for a bit I feel like I should comment on my experience with it.
So far, my experience is a postive one, based on using Foobar alone, and comparing that back and forth with Foobar using Jplay.
The first thing that jumped out at me was improved space between instruments, and between instruments and vocals. I can definitely hear an improvement in this aspect with Jplay. I want to hear an increased width and depth of soundstage, or improved placement in the soundstage, but it's not there. Instruments and vocals just sound more "seperated" if that makes sense. My setup already performed exceptionally regarding soundstage width and depth, so maybe any difference is beyond my ears?
The second improvement, not as prominent as the first, I'm mostly hearing in cymbals. They sound a bit more "real." If it makes any sense, I can hear the "thickness" of ride cybals, and fast stick work is more defined. I just went back and did another a/b Foobar vs. Foobar and Jplay... and listening carefully to the cybals the difference is actually bigger than I thought at first. Quite nice!
The last improvement I hear is a slightly tighter bottom end. Some tracks that were just on the verge of sounding boomy are reigned in, but tracks that are boomier sounding are still... boomy. A big part of this is lack of axxoustic treatment, which I am going to address soon with some DIY bass traps.
Don't shoot the messenger here, I'm not associated with Jplay in any way, I don't even own the software, I'm using the trial version currently... but I think the paid version might be my next audio purchase.