For small bookshelf speakers, I'd definitely get a sub, even if a small one like the Hsu STF-1 ($300). If adjusted properly it probably wouldn't disturb your daughter, although this would depend on your listening level and house construction.
Axiom speakers have a nice detailed sound so many people enjoy them without cranking the volume way up. Properly adjusted the sub won't be boomy or obvious like you hear from cars playing hip-hop music. It just augments the bottom end in a smooth, cohesive manner.
One possible config is M3tis up front, VP100 center, QS4s for surround, and Hsu STF-1 for a sub.
You said 75% music. If your receiver has Dolby PLII or Harmon Kardon Logic 7 to reconstruct surround from stereo, that would be great. If not you'll still have a nice stereo system. You can also examine whether your favorite music is available in multichannel surround format on SACD, DVD-A, Digital Dolby or DTS 5.1. The latter three will play on most DVD players, SACD requires a special or universal player. However universal DVD/DVD-A/SACD/CD players are available inexpensively -- the Pioneer DV-563A is about $170.
Some sources to check for surround music are
http://www.buydvdnow.com/,
http://www.highfidelityreview.com/, and
http://www.acousticsounds.com/
You can sample surround music by getting a DVD-A (which usually have Digital Dolby 5.1 compatibility tracks) and playing that on your DVD player. If you have a large stereo collection and don't go the discrete surround route, there are surround receivers with Dolby PLII for under $350.
