Oh crap. So much for a productive work day tomorrow... Thanks for all the input! In terms of a separate amp, I pretty much intend to use the headphones directly outputing from the headphone jack on my Onkyo 606. Would this act in the same way, or is there a unique headphone amp you are talking about? Total rookie, I know...
Regardless, I think if I stick with the Sennheiser I will probably go with the 595's since this is really a secondary pursuit (at the moment anyhow... audio hobbies seem to have a way of collecting), and I'd rather put the money saved towards my future towers. She can deal with any leakage... I hope.
It will be fun researching these other brands though, thanks for the push in their direction.
M60x2, VP150, M22x2
Onkyo TX-SR507, PS3 (40gb)