I am a strong advocate of finding the sound one is after by hitting the headphones first. No matter what is upstream, the overall "signature" of the headphones is going to remain. If R10's are
it! for someone, then if the wallet is able, go for it! If it is the HE90's, HP-1's, Koss 35's...whatever! $12000090 or 20 bucks, get what one likes, then go to the source and try to pump in as much cash as possible into it (within reason, I do believe diminishing returns comes in strongly at 5kish, perhaps lower depending on the design.)
I voted for both, because in the end, I believe in synergy first. RS-1/RA-1, excellent combo. Paired with silver IC's and a bright source, and ears may bleed. HD600's with copper IC's an EMP Anniversary and a warm source may induce a coma.
Once the overall sound has been chosen via the tranducers, one wants to maximize their potential. You don't want to castrate the phones straight away by killing resolution, dynamics, PRaT etc, by choosing a poor front-end.
All of the other stuff that was written concerning physics, math etc., fun read but matters are far simpler:
1) Choose overall sound (if it be a Grado, then one can expect an overall Grado house sound with the current line and have it refined to a high degree with the RS-1's)
2) Drop some serious cash into a well reviewed/received source with perhaps the potential for upgrades/mods/trade-ins
3) Amp those phones!
4) Tweak/attenuate/equalize with cables and tubes
5) scrap the whole thing and buy a cheapy system and gazlllions of cds/records, cause in the end it should be all about the music