As mentioned in previous post, I've downsized this hobby.
Previously owned Hugo 2 and Aeon Flow Closed. Bought my first "serious" audio gear at an Army PX while serving overseas in early seventies. Have been buying audio gear ever since.
Anyway, to points of this post..
Looking for punches far beyond its price small footprint desktop DAC/Amp that be great match for W900.
Still have my MSR7b (you sold yours, but like it) but if purchase W900, will sell MSR7b.
Currently use MSR7b with Zen Dac. It's OK. Coming from Hugo 2..well, you know

To my ears and depending on recording, Zen Dac treble is too hot. I use with JRiver, and really need a good equalizer profile to adjust EQ in JRiver to get this Zen Dac in a better place. The Zen Dac firmware updates I will avoid. Better to EQ with JRiver EQ to tame the treble.
Probably sell Zen Dac and MSR7b when purchase W900 and excellent DAC/Amp to mate with it. Topping seems to sell some interesting DAC/Amps. I've owned some FiiO and iFi gear in past. Still own iFi Nano One to use in small office with Audio Engine A2+. Nothing special, but good for small office computer setup.
The MSR7b and W900 have low impedance, and wonder realistically just how much improvement I can expect with a $250-$400 (my budget) DAC/Amp and W900.
No disrespect meant (I have not heard compared W900 vs MSR7b), but the W900 seems just on specs to be a MSR7b with pretty wood cups.
You love the headphone, but from what I see, you have Pathos tubes and very nice DAC to use with it.
Btw - I too love classical. After that, love many Jazz titles. Yesterday was listening for the umpteenth time to a favorite composer, Chopin. Ballade No.2 Op.38 in F - This one is in my bones till I die.
A favorite (and likely unknown to most Jazz listeners on head-fi or any other music forum). This single track expresses much of what I love about Jazz - first track Blue Stompin' from album Blue Stompin', Hal Singer & Charlie Shavers.