Hmmm. The one thing I'd say the HD-25 1 II's have over the AH-D2000's is portability. The Denon's sound amazing and look pretty snazzy but they're big and a bit bulky (as most higher-end cans are). They're not meant to be taken around with you. They're meant to sit at your desk or with your hi-fi gear but not for traveling around. Those Sennheisers, however are much more portable in nature.
I've only ever owned the Denons but I've heard the Sennheisers and I'd have to say I personally like the sound of the Denons more.
Because of that, I'd say it's all a matter of where you'er using Them. If these really are for when you're at home and you don't plan on taking them with you the Denon AH-D2000's are fantastic for the price, full retail or not. If you need something a bit more portable then definitely go with the Sennheiser HD-25's.
Whichever you choose, they'll both be a fantastic step up.