I have. They are VERY similar in some respects-
1) they are almost identical in efficiency. I didn't have to touch the volume control doing A/B
2) The mids are very similar.
3) Somehow you can just tell they are related, as in driver type, count.
Differences are-
1) The DTE500 is more reference tuned, closer to the actual recorded track, less colored. It has a wonderful clean, liquidness to the sound, where as the EST50 has more texture, more "fuzz" less accurate but fun.
2) The bass on the EST50 is fuller, and can, with some tracks, feel bloaty. It gives the presentation a richness, warmth. Not as accurate, again, but a nice flavor.
3) DTE500 has a really large soundstage, sound of spaciousness. Great reverb.
4) The treble on the EST50 is nice EST, the treble on the DTE500 is Godlike EST!
I think they compliment each other and am in love with both and would not let either go from my collection. But, if I was forced to let one go, it would be the EST50
BTW, my EST50 has the Penon Vocal on it and the DTE500 has the Null Ethos MKV.