Headphone AMP fpr Beyerdynamic DT 990 PRO

Hello forum,
I am looking for advice on a headphone amp. Here’s my situation.
I listen to music and actually play music on my PC. Audio interface is a Yamaha / Steinberg UR28M and I used to have a Sennheiser HD280 pro. This was and still is a reasonably good combo, if I have to be picky about it I would say a bit veiled and booming but overall good.

The Senns are rated at 48 Ohms and the Headphone output of the audio interface is rated at 35 mW+35 mW, @40 Ohm

In an effort to upgrade I bought a pair of Beyerdynamic DT 990 Pro, open back design, I went for the 250 Ohms model, it had better reviews and was able to get it a a good price.

Now it turns out the audio interface does not provide enough juice for the cans. At max volume still the subjective listening level is barely adequate and there’s a clear feeling hat I could / should get more out of the 990s.

So I am looking for a headphone amp to improve he situation

My requisites:
- MUST have enough power to drive them properly
- MUST NOT have a wall wart, which I hate form the bottom of my heart
- DAC on board, I don’t care either way. Probably best not to have to keep the price down, on the other hand it would add flexibility
- USB powered would be nice but I doubt it would produce enough wattage to better the situation or wouldn’t it ?
- Budget possibly within 300 Euro
- A small box would be preferable to a big one

Thanks for any suggestion / experience you might want to share !

Mark