Musical Sub Suggestions

Currently, I'm very pleased with the sound of my system but would like to experiment with adding a bit more to the bottom end. Bass now is well defined, musical but not 'substantial'. This suits me for the lion's share of the music I listen to, but occassionally I get the itch to relive my youth and pull out the hard rock stuff.

I've worked on speaker placement a bit, altough I'm very limited in this regard due to WAF. Most listenning is done in the nearfield anyway. Looking for sub suggestions to mate with speakers listed below. Additionally, I'll need to consider crossovers, so if you have any thoughts along those lines as well it would be greatly appreciated.

system is:
Joseph Audio 7si signature (MKI version) w/target stands
Jupiter2000
Simaudio I5
Zu Cable Warmouth IC
Zu Cable Julian bi-wire (double run)