Atom should be fine mate
I have both of these cans.
First of all, I'm unsure about the quality control issues, but if you are worried about it, buy new, Massdrop has a warranty for that reason.
Second, the M100s can take more bass EQ than the Purplehearts, but as far as mature headphones go, the Fostex is quality all over. Better believe even without EQ it brings that sub bass, bass and mid bass when it's there! For critical listening I even -3db the 62hz range because with bass heavy music, it can bleed into the mids a bit. Your suspicions are right though, that treble is crispy and clear and for heavy Dubstep I do nuke the highs. You can get quite a nice amount of bass out of it, but it starts distorting if you bring it up to V-Moda M100's levels. I have an EQ that is known to distort on the already very bass heavy stuff, but does alright with normal bass music.
Make no mistake, for the resolution and detail you're getting, it's definitely a massive step up from the V-Modas (and everything under the £400 mark IMO), but it's not the perfect solution. Even the 900s which have even better drivers doesn't outrightly beat the M100s in quantity.
Do I regret getting the Purplehearts? Not for a second, they are my current favourite headphone. The M100s still get some head time though for those extreme sessions >=D I highly recommend you grab a set of Purplehearts man, especially since you already know about EQ'ing. There's a bit of burn in for the first couple weeks where the very high treble reigns itself in, but after that, you're golden. If you do get them, the Dekoni Hybrid Elite pads take the comfort to another level too - but, we can get into all that if you actually decide to get 'em lol.