I really enjoyed listening to the setup last night. I agree that the DA-500 doesn't do anything wrong to these ears, which adds up to a pleasurable listening experience across a range of genres. The midrange may not be the liquid cool goodness of the 840C, but I do not think it is sucked out. In fact, I couldn't help but be slightly reminded of the DL-103. Maybe this is just the subconscious part of my brain struggling to find a house sound between two items from the same company created half a century apart, but I swear I didn't consciously arrive at this comparison. I was listening to the vocals in the midrange, and started thinking "this sound reminds me of some other source I've heard," then the DL-103 came to mind. Neat.
As for the Wheatfield, I hadn't really spent quality time with it since I eliminated my headphone rig a few months ago (it didn't get a lot of attention in the speaker room, because when I was in there I listened to, you guessed it, speakers), and I am definitely happy to come back to tubes. Tubes are so awesome. I often forget that it was a tube amp (PPX3 Slam) that first really made me a believer in hi-fi.