As background information, I'm a progressive fan (metal and rock), and between the bands that I consistently listen are Dream Theater, Porcupine Tree, Queensrÿche, Fates Warning, Opeth, Tool, Pink Floyd, Genesis, among others. In fact in my library the group of which I have the most songs is PT.
After trying some headphones (actually a lot, at least IMO) like the HD6xx, K701, Pro 900, many Grados, and a couple others I finally decided that the one I liked the most was the HD6xx family (but I wasn't sure which one I preferred). Then I had the opportunity to try both HD6xx for a month, and decided for the HD650. Finally, just before deciding to buy the 650 I had the opportunity to try the HD800 out of a Lehmann BCL and comparing it to a balanced 650 rig (mine are not balanced yet, but I plan to in the future), and chose the 650 over the 800 (Note to others, I'm not saying it's superior, it's just what I liked better). All this decisions were using progressive bands and some others (Coheed and Cambria for example) as a point of reference.
Take all the following as my opinion. So why did I choose the 650 even over the technically superior 800? Well, at least for my taste, it's characteristics are great for the genre. So here are my thoughts:
-I discarded Grados because they were too upfront, their soundstage wasn't adequate and just plain wrong for groups like Dream Theater and Pink Floyd. But then I'm too sensitive to treble so Grados have never been my taste.
-K701, paired with my source, was far too analytical and lacked bass impact (although the quality of what was there was pretty good) which made them sound thin, at least compared to the others I auditioned.
- D5000 boomy bass was too overpowering over other frequencies and made electric guitars to sound a little drowned to my taste since I found mids a tad recessed(I have heard once a Markl modded one, not for long but as an uninformed opinion is that their bass was poorly controlled also, but it was an improvement), and in the highs they became sibilant in a couple of tracks, comfort wise the loose clamp was a bad point in my book since they fell down a couple of times while head banging during an Iron Maiden listening session. So adding all those factors I disqualified the D5000.
- Now between the 650 and 600, I compared them extensively for a month. After the first week I was almost sure I would pick the 600 since they impressed me with their forwardness (compared to 650) a lot more. But then I noticed that every time I was going to hear music with a heavy electric guitar component I reached for the 650, artists like Joe Satrini or Petrucci just sounded better in the 650, and also the 600 got a little fatiguing with groups like Queensryche or PT.
Then I had the opportunity to try the 650 balanced and I found they were superior in all aspects to all the headphones I auditioned, and was really surprised by how much they improved (clearer without becoming fatiguing, a lot more speed, better imaging), and decided that investing in the 650, and later going balanced was the path to go for my progressive tastes. Then, just before buying the 650 I had the opportunity to try the 800, and technically they were superior. But I preferred the sound signature of the 650, the 800 striked me as too analytical, in some sense similar to the K701 (but they were far superior, the K701 sounded so analytical that I could say they sounded sterile and thin, not the case with the 800). The 650 transmitted the feelings of the artists in a way I liked better (and are a lot cheaper, which is huge, but even at similar prices I would have a hard time choosing). Just as a final note, about two weeks after buying the 650 I met an audio engineer that had a Pro 900, and he let me audition them. I found them quite similar to the 650, but with too much bass (which if I remember correctly, there is a mod that helps exactly this). Unbalanced it's a tough choice, the 900 is speedier but the mids of the 650 are very tough to beat, Pro 900 are also excelent in the mid region, but there was something in the treble area that made me hear the 900 as too "metaly", I can't pinpoint exactly what it was but I didn't like it. So to summarize, I think the best sub $500 bets are:
- HD650 and in then sometime in the future go balanced.
* If it wasn't for price, I would put the HD800 tied for first
- Pro 900
- HD600
- D5000
(If any of those isn't in sub $500, I'm sorry, I'm not in the states so I am sometimes not aware of pricing there, which by the way is miles and miles better than where I am) I know I am forgetting a couple of headphones I also tried, but I think what I've written reflects my recommendation. I wish you luck in your search!