Have you heard the Lambda (SB) with the SRD-7 adaptor by chance? I'm wondering how that compares to the 207 & 507.
Haven't heard it.
Specifically I wouldn't recommend these with metal. Overall they lack the refinement/speed/resolve. However, the 507 is a bit rough/forward sounding,
which I feel gives some excitement/energy metal can sometimes benefit from. That is, it's a bit harsh up top and features some mid bass hump.
the 507 sounds less diffuse in stage and offers better separation. I really feel the 507 and Electra or some other colored front end does wonders to make the 507 more engaging and less fatiguing.
The 207 doesn't really feature much harshness comparatively and I feel sounds very agreeable, but a touch boring. It lacks that excitement and overall detail retrieval. No mid bass, so they sound thinner, but still very quick. Both make most dynamics feel one-note and blurred.