I did try it a couple of times at a Audio store to audition the normal Zen Stack, the non drop versions.
When using the 3D, this just adds a little more width to the overall sound as the name implies. X-bass is not as extreme as the TrueBass on the regular Zen DAC but more like just makes it crisper. TrueBass is just bonkers and absolutely love it. When I actually auditioned the stack for the 1st time I was a bit disappointed to be honest, most likely due to pairing it with the 58x. I would imagine it would pair well with Bright headphones, since the stack will add warmth, width and keep the amazing detail.
Value Proposition wise, the Zen DAC itself is really one of the best if not the best DAC/AMP combo in it’s particular price range. As a stack it’s only worth it in my opinion if you have higher Impedance Headphones with a brighter sound signature.
I hope this explanation helps