I have had the Topping A90D for a few weeks now, really enamoured with this piece of tech.
For reference I previously used a Gustard P26 preamp, with Burson V6 Vivid swappable opamps (over $1000 AUD price tag not including the opamps at $100 AUD each) and the THX 789 ($600 AUD price tag at height of 789 mania, arrived to me on December 2019).
This thing is a better preamp than the Gustard P26 and a *much* better amp than the 789.
Layered, open soundstage that envelops the head, balanced tonality from bass to highs, *dynamic* and punchy, all the microdetails I would want and was previously missing. The staging is a little closer to the head than the P26/789 but so much more detailed and dynamic. The P26 with Burson opamps is wide but flat and not engaging, missing some detail also.
Despite the smallish, around the head soundstage, using it as a preamp to my tube amp is *so much* better than the P26 to the OTL La Figaro 339!
Dynamic and punchy, sparkly treble and thick bass hits, Large, all around the head soundstage and I have so much room for volume on tap! No matter where the volume is on the A90D the sound is not affected - this is not the case with the P26 as the Burson opamps can be quite warm and roll off the treble. With the absolute control of volume I now have, I don't have to push the tubes on my OTL amp as hard, reducing tube distortion / clipping and achieving a cleaner and much more detailed tube sound than with my previous preamp configuration.
I am currently using the A90D as a preamp to the OTL amp, at 77/99 on Low gain.
Being a logarithmic volume control, as a headphone amp the usable range is 48 to maybe 75/99, ranging from my Meze Rai Penta IEMs on the low end (4.4 balanced cable) to the HD800S (single ended cable). HEDDphone is on the way, but so is a balanced cable, so driving that is no longer a concern for this content audiophile!
Considering I paid $789 AUD (lol) for the Topping A90D, and... $1800 for the kitted out P26 / 789 combo, I can endorse the Topping A90D as it beats both of these at their own games! May be the last Topping product I get, I'm not a fan of their DACs coming from the D50 (using a Soekris 1421 DAC for a few years now) .. but I really enjoy the A90D!
P.S - I also really like the 'discrete' design aesthetic of no levers / switches on the front, and the clean LED dots for volume level and input/output selection.. very clean looking unit in my hi-fi rack..