What sub for music budget

I have a pair of Monitor Audio 3i's on the way, to be running off a NAD T750 and C523. For my electronic music, I think I will want a sub just so I can hear the bassline (usually sub-50), though it'd be nice to have for all music.

Any suggestions on which sub I should get? My budget is not huge, as is probably obvious considering my choice of other gear, but I'd like something musical.

I recently listened to some subwoofers at GG's, and only hated what I heard. They sounded inarticulate and overly deep (like they didn't play as high as necessary, and the frequency response actually went up the lower the bass got. There must be subs out there made for musical listening, I'm just not sure what their names are.
And for the record, though I listen to electronic music mostly, I like an even sound. I don't like my highs too bright, and I don't want my sub to be overbearing.
In addition to ambient, drum & bass, dance and trip-hop, I also listen to jazz, classic rock and the occasional hip hop disc.