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 !

You might look into an Xduoo XD-05 Plus you can find on Drop.com. for $220USD.  It puts out 1000mWpc into 32 Ohms, has a nice DAC, can be powered via USB and has a battery onboard so is portable.  If you can stretch or find one used an Aurender Flow is a very nice DAC/amp.  Also at Drop a little cheaper is the Topping DX3 Pro LDAC, but it puts out less power and is not portable but also very good for the price.  Hope this helps, and best of luck. 
The XDUOO looks very promising, I'll look into that, Thanks.