I have about 72 hours of burn-in time on my RP-21's so far, which I purchased from Full Compass. Sound does change throughout burn-in, but it's certainly a gradual rather than a drastic change. Here are my observations, many of which may have been posted earlier in this thread.
1. Clamped way too hard out of the box, literally made my head hurt. During burn-in, I laid them flat (with the drivers rotated 90 degrees). As others have mentioned, this stretches out the headband. After about 72 hours of burn-in in this position, they clamp just right -- strong enough to keep the closed design sealed against your head/ears, but not so strong that it hurts. Comfort is very good after this break-in procedure. It's a closed design, so fresh air doesn't get to the ears, and my ears tend to sweat a little bit from getting warm after about 15 minutes of listening.
2. Sound quality is very good for the price paid. Without quoting the dollar amount, I'll say that I paid the same amount for these as I did the AKG K240S from Amazon.com. Both are good headphones, but I would suspect the average HeadFi-er to prefer the sound signature of the RP-21 for the following reasons:
- RP-21 is more netural than the K240S
- RP-21 is more detailed, accurate, and has a better (wider) soundstage than the K240S.
- RP-21 has a Grado-like approach to reproducing guitar-type sounds, whereas the K240S has trouble keeping up with any types of fast rock/guitar music
- RP-21 seems to sound better across a wider range of music types, K240S seems to work best with slower music with strong (but slow) bass
- RP-21 sounds better without a headphone amp than the K240S
- RP-21 sounds more like an open headphone than the K240S (this is striking, because the RP-21 is closed, whereas the K240S is semi-open)
However, one should note that the sound signature does have certain flaws. I think Skylab summarized the flaws best in this thread -- most of the flaws seem to be in the treble range. Also, there may not be enough bass with a weak source (compared to say a Grado), although there is adequate and accurate bass when amped.
3. Construction seems very durable. The materials remind me of the kind of materials used in older Japanese cars. They're not the most fun to look at or touch, but they're sure durable, hard to damage, and function well. They seem to be designed with the studio user in mind, where the cans would be thrown to the side when not in use. This bodes well for the casual user, because they are likely to be more durable than we'd ever need them to be.
4. Isolation seems very good. However, I should note that this is my first full-sized closed can. They certainly have better isolation than the supra-aural Senn PX-200 (although they are generally less comfortable and heavier due to their circum-aural design). For home use, I think they have TOO much isolation. I cannot even hear my cell phone ring, and I wouldn't want to have left something on the stove by accident when wearing these. I think they would be good for use in a library or workplace, since isolation is very high. Although they do not fold into a very compact shape, I would probably not mind rotating the drivers to the flat position and throwing them into my bookbag to use them with my laptop or DAP.
Summary/Conclusion: I was very impressed with the RP-21. Without stating my purchase price, I would say they sound more expensive than they are, which is a compliment for Equation Audio. To summarize my comments above, they are not the most neutral can, nor are they the most fun. Instead, they offer a good balance between the two. They seem accurate enough to be appreciated by audiophiles, but they couldn't be considered true hi-fi headphones. Their strongest attributes are that they are closed, offering good isolation, but with a sound signature that has the feel of an open design. Most importantly, the RP-21 should appeal to value-conscious buyers like students, since they represent good value for money.
So far, I'm satisifed with my purchase, especially with the versatility of these headphones. They sound decent running out of my headphone amp, but they don't sound much worse when running out of the headphone jack of my laptop or E-MU 0404 USB. If anything, my experience with the RP-21 has shown me what a pleasant sound signature is possible from a closed design. I would be in interested in looking into more expensive closed cans in the future (to use at the library or work), such as the Denon D2000 or ATH-A900.
