Since you know and use Reaper I would render the song minus the bass track and move them to Reaper. I would use your MIDI guitar controller in Reaper. Using a DAW for recording is much easier then recording in BiaB IMHO.

Another option is to record your guitar, I am assuming you are a guitarist based on your MIDI guitar controller, playing the bass part then transposing that track down an octave. Sometimes that can sound as good as a bass guitar.

Also, again if you are a guitarist, go to Rondomusic.com and buy a bass. You can get quality instruments at a very good price there. I have a number of guitars and one fretless bass from them. The fretless bass is what you are hearing on my more resent songs and it only costs around $100 USD.

I hope this helps.

Doc-take it easy John this is just a sharp scalpel. It will not cut deep so don't worry.
Me-I'm not John
Doc-I know, I am!

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware