Senn HD600 headphones = what Speakers?

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.

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.

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

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.

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.

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.