I usually don't like to characterize a certain speaker as better than another for certain types of music, but there are rhythmic, dynamic and tonal considerations that could shade a speaker as less suitable for for a given music style. Basically any speaker that would sound good on rock will sound good on rap. Search the archives, it's been a well discussed topic.
If you want to hear it the way they listen to it in the recording studio, I suggest the following:
Dynaudio M1.5 or M2 (click on the link for Model Overview and scroll thru the PDF0
PMC EB1i - there's a pair listed on Audiogon
Quested VH3208Along more traditional audiophile lines I think your Legacy idea would work. Also certain models from Wilson, NHT, VMPS, Revel and Zu could be considered.
If you're serious about the really frickin' loud, then seriously consider the pro models. There's consumer loud (105-110dB) and then there's pro loud (120-125dB). That's more than twice as loud what most people think of as really loud.
Let us know what you end up with.