@strokerace, just tested.
Buy the WS99BT and never look back.
They sound much better both wired and wireless.
Much more transparent, more highs, more balanced, still with same powerful bass (may be, not sure, too early to say but may be that the WS99 has got a little bit more bass, but it is a much worse bass, less controlled, less natural, muddy, boxy.
And the WS99BT can get louder, in BT mode.
I have tried them both with the HM5 Earpads (a must have with these both models, maybe more for the WS99BT which have thinner stock pads and are therefore a little bit less comfortable without the HM5).
I did not have two angled pads, so I have alternated, a bit the angled on the WS99BT and the flat on the WS99, a bit the opposite. This is because the flat give a slightly more boomy low region and add a little muddiness.
And result?
The WS99 were muddy and veiled even with the angled, while the WS99BT were transparent and not muddy even with the flat.
Go for the BT, absolutely.