Well, i think Classics with my modded Hybrid sound really great i love them, maybe i like it more than with the original earpads(i think mine are 2nd gen pads, so bass is quite boomy), the downside(?) is just flaws in bad recordings are more exposed now because it sounds clearer with a little bit more mids and highs.
Sheep Skin should sound great without mods, as there are many recommendations about it.
I'm not really interested in YAXI, too expensive IMHO.
Just put some foam cover inside earpads, i suspect the problem is with Brainwavz' thin foam cover and it's size(it's designed for their HM5 after all), thicker foam cover should improve bass and tame harsh mids/highs.
it's similar with Senn HD558 foam mod, except it's the opposite of that.