Just to get back on topic:
I rarely comment here, but with a topic like prog rock/metal at stake, I feel like I must throw in my 3 cents into the ring to help with the decision-making process.
To give you some background: I am a drummer and I need to be able to hear the beat to get a feel for the music; I also am a huge fan of bands like Rush, Dream Theater, Porcupine Tree, Between the Buried and Me, Gensis, Opeth, etc. I own the Senn 600's, 800's, and the Denon 7000's. I have listening experience with the Sony R10's, AKG K1k's, Denon 2000's, and various Grado's ranging from entry level to high end.
I agree with the previous posters who argue for headphone over amplification over source; ideally one would have all three but this world can be anything but ideal. I have an "uber" rig at home ending with the HD-800's and a cheap rig at the office with the HD-600's/tiny combination amp/DAC. I have perfect hearing and to be honest the HD-600 rig is 85-90% what my main rig is. It's nice to have the peace of mind that I would need a 10k investment for any audible improvements, but that's about all the extra money is good for.
Soooo, you're wondering how does this all relate to prog rock? The HD-800's and more expensive headphones (with a good DAC) are going to do a better job with microdynamics. You will hear the underlying bass drum beats, the feint symbol accents, recessed snare rolls, high hat dynamics, any background keyboard melody, and the interplay between the complex instrumental structures more clearly. The high end combination will sound slightly better in almost all regards, but in an average prog listening session you will mostly only notice an increased precision in the presentation. With my HD600's and cheapo amp/DAC I can still feel the music, tap my foot, and appreciate it for what it is. It sounds really really good. It is implicit that with being a drummer I should like bass, but I would never ever trade my 600's for any Grado's or even the Denon 7000's with a better amp/source for prog rock.
The amplification does far more for prog rock/metal to make it listenable and enjoyable than a DAC in my opinion; prog rock isn't about an nnn tss nnn tss beat, it's about subtle dynamic and beat changes which a good amplifier is going to bring out more than a DAC. Microdyanmics != feel of music. If you argue otherwise, then how did these bands get popular in the first place if you can only feel their music with a high-end system? But for headphones? I would go with the Senn 600's any day of the week. Prog rock relies on instrument interplay more than bass, and thus need something that can present it well. I could see how someone could make an argument for a pair of high-end Grado's accomplishing this, but I think it's overkill for what you need to get into the music, and their upfront presentation gets in the way of the enjoyment for long-term listening sessions.
Any way you choose, however, happy listening