'Better' is a very subjective opinion, so I'll hold off on deeming it better (or worse) than other phones. They're certainly similar sounding phones, though.
I haven't had the 004 for some time but I remember its sound quite well. The RPM & 004 are both quite balanced in sound signature.. the 004 was a little warmer through the lower midrange/midbass. That warmth could creep into the 004's vocals sometimes too. The 004 might have had a slightly deeper soundstage while width between the two seems about equal (the edge going to the RPM by a hair though). The 004 imaged a little better. I find the RPM to be better controlled from end to end.
The RPM has a tighter, more linear bass response. The 004's bass could sound loose & bloated and at times, especially when unamped. The RPM's mids are a tad smoother & (comparatively) drier. It's still lush & full sounding, just not as wet as on the 004. Mids seem to always shine on the any phones in FA's Master series.. no exception on the RPM. They're lovely in their nautral tonality, smoothness & detail. Treble texturing is quite similar between the two but the RPM has more air, is crisper, possesses more sparkle, and is more detailed.
I'm still letting these guys burn-in.. but they continue to get better & better. I'm really impressed with the sound quality they possess for the price & from an on-ear (it seems tough to get true hifi sound from on-ears.. I think FA got it right though). I want to stress GIGO as I did in my initial post. Feed these HQ files. Low quality recordings/digital files will reveal themselves quite easily & for the uninitiated, my think the phones sound bad.. NOT SO!
Not sure if I'll keep the RPMs though since I've got enough phones as it is & my customs (and HE-500 get most of my listening time). I definitely want to let them run in for at least a hundred hours or so, though. I bought these cause I'm a big fan of Fischer Audio and just had to hear 'em! Even if I sell them & lose a little bit of money on them, I'm more than happy I got them.. great phones... that I would take over the 004 .
Zio currently has the 004 so he may be able to offer more in-depth comparisons.
Edited by FlySweep - 2/18/12 at 2:55am