Just to clarify, I'm sure you're aware of this but for the benefit of those who aren't, there is a difference between the ZDSE and the ZD and they're not completely synonymous. The ZD is the older version with a fixed output impedance of 12 ohm whilst the newer/current ZDSE has a 3 ohm/12 ohm Zout switch. I sent my ZD to a tech to mod it to make it into a ZDSE, with great thanks to Craig Urthus on what was need to bring down to 3 ohm.
The tech who's doing the work is further making some tweaks to bring out the most of this ZD(SE) variant to suit the TH-900 and last weekend we were just doing the finishing touches to trying different cap values for the negative feedback. Sadly I had to leave the shop prior to completion but let's just say that (after 2 months!) I'm pretty happy with the results. I don't think I can be more pleased with the overall configuration which seems to bring out the best out of my TH-900 for my tastes. I can't give any detailed impressions since it was tested with temporary alligator clips on the caps & resistors to try various values but the words "full and robust", and "holographic" presentation comes to mind.
Prior to the mod, the bass of the TH-900 from the headphone out of the Invicta sounded more correct in comparison to the ZD's 12 ohm Zout. After the mod, the ZD(SE) variant gives out a fullness bass and mids.