I recently acquired a Fostex sub woofer and connected it to my Roland Street Cube EX. WOW, what a great sound! So much better then just the Roland on its own!
But I don't use the pre amp stuff of Roland's, because it muddies down the tone too much. Instead, I have a little pedalboard with two Sotic SP compressors, one for each side's output (setting the EQ pin on the melody side compressor to cut lows, for less cross-talk from the bass side's bass strings) and then the two sides are summed into the two stereo inputs of a Strymon Big Sky reverb (with amp/cab sim activated). This is then lined into the stereo line-in of the Roland Cube Street EX.
I also have a Tube Screamer and a volume pedal on the melody side. And I just connected a CV expression pedal to the Big Sky to control Eno-styled "reverb cloud freezing" behind my playing. What fun! I really want to recommend checking out an expression pedal with the Big Sky.
This week I'll use this little "big sounding" rig a solo concert for a healthcare institution and a few days later at an art exhibition opening. A very handy little portable "rig on a shoe-string"... I think I'll bring it to Paris in February for a couple of gigs down there. I
Maybe Roland wants you to connect another Cuber there, I don't know. I'm fully cranking the sub and its experienced volume nicely follows what I set the Cube's "line output" knob for.