Yea, that's what I noticed about the amp. Compared to the Hifiman EF2A(stopped working after a few weeks) I had in the past, it's really flat and sterile on the DT 990s. Really good for warm and dark headphones though, seems perfect for darker or more neutral Senns. It can be a little harsh in the treble on the K240s at times due to the K240s 10khz spike, but the spike never causes me fatigue on the K240s and doesn't bother me much, but I would like a tad of the edge taken off. I would like an amp that smooths out the treble more though. Any suggestions on an amp that smooths out and tames the treble?
I do like my Audio gd C2.2 it has a nice relaxed tone and nothing seems emphasized including the treble. It also sounds quite airy with good soundstage for a SS amp. It comes with stock burr brown opamps that were a little too warm for my tastes but I replaced them with the much better opa627 and the treble is now very detailed and sweet with no hardness at all. (thats using the HD800's aswell) I've had quite a few amps come and go while I'm hanging on to this one. Quite a bargain for what you get. If you do look into it, makes sure its the C2.2 as opposed to the C2 as there where improvements including output power. If I remember rightly it gives about 3 watts into 32 ohms.
Another bargain is the good ole' M-stage, I use one for my bedside rig and this also has nice treble. Not quite as smooth as the Audio gd but defo not hard sounding either. the M-stage is more suited for higher impedance hp's because of the lower current output (the exception is the K701/2's). Where as the Audio gd can cater for both high and low impedance in my experience.
Both amps can be fine tuned with different opamps. And remember, a very subtle difference can make or break a component! IMO :)