After a little more digging, it seems that the Music One might not have as much attack as I want, as it seems to be a very smooth signature.
I've been perusing the 1964 catalogue, in particular, the V3 and the V6-S. It looks like the V3 and M1 are completely different beasts in terms of signature, where the V3 is a musical but aggressive while the M1 is more reference and laid-back. The V6-S looks to be in another league entirely more akin to mid-level customs, with a reference sound with slightly tilted bass.
I'm ultimately looking for something natural and fun, but not colored to the point of a fatiguing/tiresome signature over a period of time, and suitable for metal. Does it look like the V6-S is taking the cake?
The V6-S seems like a no-brainer if I'm perceiving it correctly save the hefty price. The V3 is tempting with its cheaper price tag and musicality, but the perceived boominess is a little scary, especially when it comes to light-speed double bass on some death metal tracks, and its sibilance-proneness. The Music One (Two) is also valid because though it doesn't have the attack of the other two models, it has a fun, textured low-end and smoothened response that is safe for metal tracks.
I suppose I'm just thinking out loud here, but would you say that the 6-S is worth the investment in this regard, or perhaps you'd like to point me towards some other customs/universals closer to the V3/Music One price point? I'm looking at customs mainly for the comfort, isolation, and aesthetic.
I really couldn't find fault with the V6-Stage beyond the very mild sibilance I encountered on some tracks. For the price it's as good an earphone as I've heard. The V3 is too bassy for me personally and the Music One, good as it is, still has some limitations in bass and treble (which the dual-BA Music Two may remedy).