I've had the DPX21 for a couple months, I hated it very much.
It does not have big earcups, in fact they're supra-aural. Even though clamping force is very minimal, I personally had a pressure point on the top of the headband, and the supra-aural fit with the vinyl pads made my ears ache after a while. The comfort was not very good.
Other than comfort, the sound was abysmal as well. Not only does the DSS hiss, but the built-in proprietary inline amp of the PX21 hisses as well, this makes for a super hissy product. Their bass is there in quantity, but it isn't very impactful, and overall the sound is rather muddy with a subpar soundstage.
AD700 would fit all of your bills, but it's not bass heavy.
That Samson headphone with an earforce DSS would most likely be your best bet. If you had the ability to get the Mixamp online, I would really get it.