I use a GK 400RB-III 210 combo (like the Backline, but slightly less power and plywood instead of MDF cabinet) for rehearsals and some performing with an external 15" cab.
I've also had a couple of different 2x10 cabs over the years.
I'd like to suggest that maybe you're just asking too much of an amp that size, or put better, one in that power class and driver class. It's not an Eden Metro or SWR Redhead, or one of the larger GK combos, for sure. You cannot simply put it in a scenario to compete with a band, and decide you want it to really boom by boosting the low end on either the amp or an active bass, and get what you might be hoping for.
Adding a single 15" would be a good move. The GK amp has honest wattage ratings, and with the extra low end the extra cab would provide, it might be a night and day experience in this case.
But also, examine what you're doing with the controls, bass and amp. Try running the EQ nearly flat (although flat on the Backline is thought not to be all knobs nooned -- check with TBer Fdeck), and the Master (I think it's called this on this model) higher than the Volume (the first gain control). Also don't run the Boost over 12:00 for starters. I use active basses, and I always have the volume on them wide open, with EQ generally flat. Set it like this as a "control" element, then bring up your amp volume, and see if it gets it any closer to what you seek. You may get more clean volume, with adequate low end, which will seem like it's low end heaven compared to where you were before. But then again, perhaps not. We're testing the waters, trying to optimize what you've got.