I think you could do 5.0 like this:
Select 5.1 output in DSP Studio > Output Format, with JRSS enabled
Set DSP Studio > Output Format > Subwoofer to Silent
Use DSP Studio > Parametric Equalizer > Mix channels to fold anything on the subwoofer channel into the mains
Finally, I would recommend 'Internal Volume' so that you have extra headroom for the bass you're adding to the mains
If you wanted to get fancy, you could even consider using Media Center to handle the triamping. I think you'd get finer-grain control and better quality than you'd get with hardware crossovers.
Good luck, and let us know how you fare.
( p.s. I don't know if we can let you keep the handle 'Bassman' if you don't want bass down to 20Hz
)