I own an HF-2. I've never owned a 225, so I'll stick with what I know, notwithstanding the tremendous amount of positive buzz regarding the 225 as the best Prestige series headphone Grado has. I've owned the 325i, the RS-1 and the GS1K as well as my share of SR60s, SR80s and iGrados.
The SR80s were my first Grados, followed by the 325i. I like the RS1 and the GS1k for different purposes. The RS1 has a sweeter midrange; the GS1k has the wider soundstage with the jumbos and just plain slamming bass with bowls. Still, the HF2 has the best bass I've ever enjoyed from a Grado. Like the GS1k, it has recessed mids, but nowhere near as sharp/traumatic as with the GS1k. I like my mids slightly recessed, so there's room for a moderate EQ smile (full bass and HF sparkle). With the jumbos, the GS1k has too much of a drop in mids for my blood. You get much of it back with the bowls. The HF2 has enough similarity in that area to have provoked rumors that it's really a GS1k without the wooden overcoat/mushroom top. Others have called it a baby PS1k.
For classical, jazz, acoustic and other genres involving a lot of natural instruments, the RS1 is (in my opinion) the best Grado, except perhaps for the PS1k (which I've never heard). Natural instruments tend to have an HF roll-off that rewards headphones which emphasize mids, upper-mids, and lower treble. Stated another way, Grados are bright, unfiltered, open-air cans that do a great job of covering the higher end of the spectrum. On rock, heavy metal and electronica, the predominance of HF can leave listeners overwhelmed. For these styles, Grado needs a good rock-n-roll can, one with enough HF restraint to provide a balanced presentation - particularly when the phones are used unamped (My M^3 with variable bass boost could make any Grado a rock-n-roll phone).
Grado has two such cans, maybe three. The first is the old-reliable SR60, particularly with flats or comfies. With minimal ear/driver distance, the SR60 packs an amazing wallop and doesn't get overwhelmed by all that siss. The second (if the buzz is accurate) is the SR225, headphones I've never heard, but a pair whose fans rave about as the best value Grado has going. $200 is not a lot to pay for a decent set of headphones.
The HF-2 is, in my opinion, the ultimate Grado rocker. It has more refinement than any of the Prestige series cans I've owned and it has the most thump and slam of any Grado I've ever owned - the GS1k included. For all I know, it's a GS1k without the wooden overcoat. If so, it's tremendous value for the money. It isn't as rich in the upper mids as the RS1 but it's a great step between the RS1 and the GS1k, in terms of flat vs. recessed mids. It's a tribute to what you can do with a Grado, short of amping it. If the K-701 could produce bass like this, it would die from delight. The HF2 doesn't have that luxuriously wide soundstage, but what it lacks in soundstage, it makes up for in balance.
