I would expect them to sound fine, except for some limitation in how loud they can get. You should be able to get noticeably louder that a THX qualified movie theater, 110dB even in the deep bass by my calculations, assuming the 91dB/mW is accurate.. The new transformer should get you something like another 12dB in the deepest bass, i.e. 122dB. THX qualification is 105dB.
As I said this is calculations, not experience. Headphones are funny, and headroom seems to be more audible than it is with speakers - perhaps it's the missing chest compressions of live deep bass. But I offer my calculations for what it is worth. Doc B has much more headphone experience than I do, and I expect he will chime in here at some point, assuming he is not snowed in today(!)