Got the DT990 pro 250. Out of the box, the bass was disappointing (compared to the DT770 pro 80). It seemed less controlled, loose and boomy, not nearly as punchy, tight and detailed as the DT770 pro/80, and didn't seem to extend as well. I had already returned the DT770, so I didn't have them to compare directly, but it was certainly not what I expected. However, I let them burn-in for a couple days and they seem to be much better. I still like the bass of the DT770's better, and I do seem to prefer the isolation of the closed headphones as well (not to mention the straight cord...the coiled cord on the DT990 is a PITA and will be replaced ASAP). But the soundstage and instrument separation seems to be better with the DT990's, and with a little EQ the bass really woke up after burn-in, though still a little on the loose side. Plus, I'm running them off an E17, so more power could be better and just what they need to "wake them up" as all the reviews I have read have had them hooked up to decent amps. With a 1kHz sine wave @ 0dB I measured 2.95V at the output with volume set to 50/60 on 12dB gain with no audible distortion. Assuming 250-ohm impedance (which varies wildly with music obviously) that comes out to ~35mW. That's not 100% accurate since I didn't measure the current draw to get the actual impedance at which that power was put out (didn't have anything handy at the time), but it is about in line with what I've read. I'm thinking of getting an E9 (or E09K) to pair with the E17 to wake these puppies up. I was thinking O2 as well, but I do like the docking capabilities of the E9. It has a higher output impedance but that won't be a problem with the 250-ohms headphones, and anything that would be affected by the 10-ohm output impedance would run easily on the E17 anyway. My 64-ohm JVC HA-RX900's get stupid loud (they're 106dB/1mW sensitivity too, so easy to drive) running on the E17, and has really woken them up quite nicely (still no competition for the DT990's though).
As for the E17, just my luck, and the exact reason I don't like to buy online for electronics, the one I got was a dud. It would turn on with USB plugged in, but would not power on with USB unplugged. I let it sit and "charge" for about an hour, and same thing. As soon as the USB cable was unplugged, the unit would turn off. Whenever I turned off USB charging in the menu it would instantly shut off as well. The red power LED was flickering really bad and the display had some flicker as well. The battery indicator showed it was full. But with USB the unit did work, and quite well at that. Not wanting to go through the hassle of returning, swapping, etc etc, 1-2 week resolution ordeal (and since there was no "warranty void" stickers covering things up), I decided to open it up. I probed the battery leads and got ~0.7V with USB unplugged and 4.13V with USB plugged in, but as soon as I unplugged the USB cable the voltage plummeted to 0.7V. Obviously it wasn't holding a charge. I checked continuity of the positive and negative pads on the PCB (negative to chassis ground and positive to a large cap sitting in front of the dock connector), and sure enough there was no problem there. I desoldered the battery and plugged in the USB cable with no battery at all, and same symptoms including the full battery indicator. Obviously the battery was DOA (which I've seen quite a bit, so not really Fiio's fault, though it should have never passed QC). I went up to my local Batteries Plus and picked up a similar size lithium-ion polymer battery pack rated @ 3.6V 1300mAh, the stock one was 3.7V 1500mAh (googling the part# returned no results, Chinese or otherwise). I soldered it in, and it powered right up. The battery indicator now worked, along with a nice bright red LED around the power button, and the screen flicker disappeared. I let it charge up, and everything is working great, although being 1300mAh will make the battery not last as long, but it shouldn't be too bad. I tried searching on Mouser, Digi-Key, Google, etc etc, but it's difficult to find a battery of those same dimensions and capacity (or larger capacity), as anything larger simply won't fit. The battery is as big as it's gonna get, especially the thickness. There is barely enough room for what's there. There was some double-sided foam tape securing the original battery so it didn't move around, so I replicated that with some double-sided foam gasket tape I had laying around, worked fine. But the battery was an extra $21 expense I had not accounted for. I could have sent it back to Amazon no problem, but me being the impatient (and technically inclined) person that I am, I decided not to wait.
The amp/DAC itself is running great, and is a definite improvement over my Denon receiver and laptop's onboard audio. The soundstage is wider, and the mids/highs are clearer, though the Denon seems to have more bass but it's a bit boomier and not as controlled sounding compared to the E17. The E17 gets louder than the Denon AVR-1712 headphone out, and stays clearer at those higher levels as well. It drives the DT990's pretty well (though there's room for improvement) but it drives my 64-ohm RX900's to ear shattering levels! When connected to my LG G2x Android smartphone, the sound is much louder and is much better. More bass, clearer overall sound, and a definite overall improvement. I'm happy with the E17 and will definitely keep it. I just hope nothing else goes wrong with it.
Edited by c00lkatz - 11/23/12 at 12:25am