I've got a pair of 440's myself. Great headphones, can't beat them for the price. MY wife stole them a while back and uses them with her pc. I still can't get over how good the Denon's sound with the A-100.
I received my A-100 today, put in the jumpers and fired it up. With the Massdrop HDXXX's it sounds like it's overloading them a little bit, maybe 300 ohms isn't the right match and the 600 ohms would've been better. Probably should take the jumpers out with them? With the HiFiMan HE400's it brought them to life and they sound much better. Very good match with the HE400 cans using the jumpers. Also could be that the amp needs to break in for a while. I can also hear the low volume channel imbalance that I've read about.
Be a bit patient, and give it time to break in. I use my HD650 with it, jumpers on, sounds fantastic.
Unfortunately the massive gain on the A100 isn't quite a good match for that kind of planer (i.e. a fairly efficient one). The A100 is a perfect match for the "classic" hifiman headphones (HE-5, He-500, etc.) The pot in the a100 won't have enough low volume play with the efficient ones, and like you found the imbalance will pop up. Maybe throwing a preamp in front of it, or we used to use resistors as well (either in the RCA inputs, or the speaker outs).