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.

Depends on your source material, associated components, and room. I have 600's, real nice cans. I have two systems that sound better, but they ought to given the price differences

It definitely depends on what you use to drive your headphones or the room speakers. The HD600 really begin to shine with even a relatively inexpensive entry level amp such as those produced by Headroom. Room-speaker interations are also a factor that make comparisons and generalizations difficult. Properly driven, few speakers under $4,000 (even if they are optimally driven, and set up properly in a decent room) will surpass the headphones.

I use an ASL MG OTL headphone amp. I replaced a new production Sovtek 5751 with a NOS cryoed Mullard 12AU7 yesterday and really opened up the Sennheisers. If you want the sound of the Sovtek 5751 you could get a pair of Meadolark Kestrels. If you want the sound of the cryoed Mullard 12AU7 you'll need Merlin VSM-Ms.