I can't help you on the looping stuff, but as for me, I use a separate little cheap mixer (behringer) as the headphone amp and run the bass and a line from the computer for practicing with music from the computer. There are many headphone amps out there that you can just use the line in jack as a line from your computer and use it as just a playback device, like a big Ipod. That will be much better than trying to run the bass through the computer using software and an interface.
I actually use a cheap headphone "Y" cable from the speaker output of my computer. One side goes to my desktop speakers, the other to my little mixer. If you need cheap cables for this check out
www.monoprice.com. The cables are super cheap and would be decent for this kind of application if you are on a tight budget.