PurpleAngel, thanks for the input. Haven't done the mod for the HD558s because I'm not sure if I'm keeping them yet. Not bothered by the mids and highs though for them; just want more bass. Just trying to figure out if I should switch them out for DT770s given the type of music I listen to and for movies. Does the DT770 sound compressed or in your head? How would you rate it for movies? I'm guessing the bass would be great for explosions, etc. but does it work against you for scenes with a lot of dialog? I'm certainly OK with having mids and highs take a small step back in exchange for more bass (that's what the PAA-1 Pro's signature is like) as long as they're still clear and not completely overshadowed.
By the way, does anyone know if there are any shops in the Bay Area that carry the DT770s that people can audition, ideally in the south bay?