What music genres is the Stax sound best suited too?
I listen most to rock and metal.
I think the "Stax sound" is too wide of a playing field to attempt to define it to certain genres - i.e., the differences within the Stax line of headphones are such that where one Stax headphone might excel at certain genres, other Stax headphones do those genres and more, while some headphones again are weaker in those genres but stronger in others.
The following is all my subjective opinion.
My belief is that the most well rounded headphone of the Stax lineup is the SR-007. The reason they are the most well rounded is because they are the headphones which are the least fatiguing(less treble energy) and have the most body of any stax headphone. After the SR-007, the SR-009 follows in terms of versatility.
The SR-009 has body similar to that of the newer lambdas, but a lot more than the earlier lambdas. The SR-009 are a bit less well rounded than the 007 because of its treble energy, making them starkly contrasting headphones in certain genres, especially classical music with violins/cello etc. My personal inclination is that the 007 are better for long term listening use, while the 009 is better in terms of bent forward, intense listening. Simply because I find the sound more immediately engaging. The differences in resolution etc. are very, very small, however I think most people will find the 009 more immediately impressive in terms of detail resolution etc, however once you stop listening for the minutae in every track and just listen to normal music, the differences are completely nonexistent even between the 009 and the lambdas.
I think the 007 is the most well rounded headphone, closely followed by 009 and new lambda lineup. The 009+SR-L series sounds great with almost any genre, however they are exceptional with classical music, acoustic music, ambient music, instrumental (like post rock) music and similar. The 007 loses out in these genres, but are clearly better to my ears when it comes to heavy metal and its many sub genres, hip hop, bass music etc. Many hip hop / bass tracks have snares that are produced in such a way that they may become sibilant on other stax headphones, and bass that are only done justice via the 007. The added body and heft of the 007 makes intense metal music like meshuggah or similar more engaging and less fatiguing. When it comes to more middle ground musical acts like Opeth, Porcupine Tree and similar prog, it is pretty close. I probably still prefer 007 for these genres.
Moving on to the new lambdas vs older lambdas, my experience is that the new lambdas are very much developed in terms of how well they do with genres overall. The new series are much more "all rounders" than the older lambdas, which to my ears were a lot thinner sounding, lacking body and soundstage in comparison. Overall resolution is probably similar, however my experience was that the new lambdas felt a lot more "grand" and with more "depth" in terms of soundstaging. The L300 is in somewhat of a middle ground, being a clear upgrade from the older lambdas yet a bit more "thin" or "narrow" sounding, likely because of the ear pads. NOTE: when I say "older lambdas" I mean like pre 2000 era. I haven't heard the 507 etc.
To answer your question directly:
If you listen to rock and metal, I would very much recommend you get Stax over any other headphone sound. If you told me you were ONLY listening to heavy metal, I would say either get a SR-007 or a planar like the LCD 2. Not that they are similar sounding, however for death metal in particular, I think the Stax sound overall lacks body and heft to make it sound perfect, with the SR-007 being the exception.
Since you mention rock, I think going the way of Stax even if you get a 007, 009 or lambda is a good decision. Even the older Lambdas does a good job with rock, and no Stax are "bad" with any of the genres. Certaintly better than most if not all dynamic headphones I've listened to.