Curious about how the 507 compared to the regular old 404 I had. If it fixes the issues with the 404's mids then I'd be all for it.
The 404 was bright but certainly not dynamic bright as has been said. Stax stuff is generally close to neutral, though I've yet to hear something in all of headphones that's actually well and truly neutral.
less of a mids peak, better bass extension and no mids bass peak (507). 404 is far from neutral. 507 is closer and probably somewhat dark to some people.