Thanks for the reminder ;). I adjusted the bass valves on the ASG-2 to my preferred level (just barely above fully closed, about a third of the way to the first notch mark) and listened for a couple of minutes to readjust to the sound signature. I adjusted the volume on the DX50 and the gain and volume on the UHA-6S MkII so that I could switch back and forth without having to adjust the volume each time. When listening to the DX50 from the headphone out, I unplugged the lineout to the Leckerton and listened from the HPA on the DX50. When listening to the DX50/UHA-6S combo, I went from lineout of the DX50 into the input on the Leckerton, and then to the headphone out on the UHA-6S. This way, I used the same DAC in both (the DX50's) and changed only the amp.
I went back and forth a few times and took some notes, but the differences were pretty clear: the biggest gains were in the treble, where the sound became more open and significantly more real (cymbals really make this clear for me, for whatever reason) with the Leckerton as the amp. The midrange was less affected than the bass or treble, but there's a noticeable increase in the texture on guitars and the ability to delineate the different sounds at play. Vocals sounded pretty good either way, but on my quick listens, there seemed to be a greater sense of clarity with the Leckerton. As for the bass, the gains were more substantial than I'd remembered. The DX50 HPA seemed to have a greater quantity of bass, but the changes in quality were pretty obvious when moving to the UHA-6S. On the DX50 by itself, bass was pretty woolly, bloated, and poorly defined. Everything just sounded significantly clearer when bringing the Leckerton into the mix. Basslines were much easier to pick out and follow, and there was no bloating or bleeding into the midrange (which wasn't a huge problem with the DX50, but it was definitely there).
So the conclusion is pretty simple: the DX50 + UHA-6S MkII > DX50 solo, in pretty much each region of the spectrum. Now, the provisos: I did this comparison over a few minutes, and switched back and forth quickly, but this was by no means a scientifically thorough examination. (I did a quick attempt at volume matching, but I don't know that I'd be able to give any indication of how accurate it was). I feel comfortable that the results I've reported are accurate for what I heard, but I don't claim that my ears are any better/worse/sexier than anyone else's, so if somebody else has a different finding, more power to ya! Also, please note that the differences were readily apparent only because I was switching back and forth-- when I used the DX50 by itself with the same IEMs before I tried it with the Leckerton, I very much enjoyed the sound, and the issues with the sound were in no way glaring.
Summary: in my opinion, the DX50 is really enjoyable by itself, but is clearly better when the Leckerton is doing the amping.
Hope this helps! :)