If nothing else, it would still be fun to fool around with sub, while you're saving for a bigger rig. In order to make this work properly with the Promethean, you'll need a mic preamp or a mixer, because you need a way to boost the output of your bass upstream from your amp.
The normal way to integrate a powered sub, is via the sub's internal crossover. Your amp gets used like the powered satellite speakers in a PA system, with the sub handling everything below about 100Hz, and your amp handling everything else. You do it by sending the preamp out from your amp to the "full range" input of the sub, and then returning the "high pass" output of the sub to your amp, via the power amp input. System volume is controlled via your amp's gain control, and you balance the amp against the sub via the master volume controls on each.
Unfortunately, the Promethean doesn't have breakout jacks. Without them, there's no way to get the high-pass output of the sub's crossover back into your amp. You could run both the amp and sub full range via the Promethean's DI out, but the sub would drown out your amp, and make the tone downright flabby.
Thankfully, if you do have a preamp or a mixer, you can still get the job done:
1. Turn all volume controls down.
2. Plug your bass into the preamp.
3. Use an appropriate cable or adapter to connect the preamp's (or mixer's) output to the full range input of the sub.
4. Use an XLR/1/4" adapter to plug the sub's HP output into the instrument input on the Promethean.
With everything connected, gradually increase the input gain on your preamp, until you can see a good signal. Dial up a moderate amount of output on the preamp, then increase the volume control on the sub until you hear some lows. Last but not least, gradually increase the instrument input on the Promethean, until you get a good balance between the sub and your amp. From here on out, the output control on the mic preamp/mixer controls your system volume.