Senn HD600 headphones = what Speakers?

While I have not heard the headphones that you speak of, I would vote on the InnerSound Isis for three grand. When setup correctly many admirers compare these speakers along with their larger brothers the ERos to wearing headpones.

The Sennheiser HD600s are a great headphone deal (at Audiogon prices). However, I prefer listening to my Meadowlark Audio Heron Hot Rod speakers. Used the Heron i (current model) would fall in your price range. New, you might check out the Shearwaters, Kestrels, or new Swifts. I have not heard them, but the postings here have been favorable.

I have a HD600 and Merlin VSM speakers (2nd hand for around 3k). The HD600 does not even come close. In other words a 2.5k pair of speakers should outperform your headphones hands down.