I don't have a bass amp but I have two basses that I started playing recently. I own guitar rig 3 and use that for my guitar and have been playing bass through it. I have a nice set of Cerwin Vega bookshelf speakers that double as my 'monitors' and they get a decent bass response. Would adding a dedicated power sub bring out some more of the guitar rig output when I'm playing bass?
Lefty Strat Copy w/ GFS Lil' killer rails + Crazy wiring
Lefty Kona Thinline Acoustic/Electric
Righty-to-Lefty Silvertone Bass
Righty-to-Lefty Memphis Short Scale Bass

Tascam US-428 USB Interface Mixer
Guitar Rig 3