There seems to be a big jump (not ignoring the db scale) from moderate to fairly loud with those headphones:
100 dB SPL 0.79 Vrms 3.16 mA 2.5 mW
110 dB SPL 2.51 Vrms 10.04 mA 25.2 mW
The M3 can manage about 30mW into 250 ohm with 1.92Vrms so you are kind of hitting a limit potentially at the upper end of reasonable.
I think many headphones see a big jump in power requirement when going from 100dB to 110dB. Seems it's roughly around 10-11x more power needed to go from 100dB to 110dB for the ones I've looked at. HD600 go from needing 6mW to 66mW. Goldring D150, which are 32Ω, go from 3mW to 32mW. Denon D2000 go from 0.26mW to 2.5mW, and only 8mW for 115dB, compared to 210mW for that of the HD600. A potato could literally power the D2000.
30mw @250ohm is pretty puny, the Schiit Magni 2 manages 260mw @300ohms, though that is an amp only.
The Magni is mains powered though. You won't get anything like that from USB powered. Just looking at the FiiO E10K, the M3 has got around half the power into 32Ω. When it comes to 250Ω though, it's a different story it would seem, according to someone on Head-Fi who calculated that the E10K outputs 27mw, which is around the same as the M3. Whether his calculations are accurate though, I don't know.
Gain will be significant here and it seems that SMSL don't say what gain they use for their amps, which doesn't help. E10K has a high gain of 8.8dB, which is likely a fair bit more than the M3 if the OP is not getting the desired volume with around 30mw into 250Ω with his DT990's. The E10K does a pretty decent job of driving the HD600/650 on high gain, which demand roughly 2.5x more power that the DT990 needs. The extra gain the E10K has over the M3 is likely the difference.
Soundblaster Z, if the OP wants sound positional effect stuff. If not and he wants just stereo without effect stuff, E10K is a good cheaper DAC/amp option; maybe SMSL M6 if he has a bigger budget, which has around 4x the power of the M3 for the higher impedances.
