Lots of really good advice about bass setup, EQ and plucking attack in this thread. I agree that 90% of the action lies in those areas.
That said, the right rig is still important when you're on a search for a good low B.
Granted, cost is a factor, but if you're comparing pound-for-pound, extended-range cabs from people like Greenboy, Duke LeJeune, and Andy Lewis slay the usual stuff from GC.
Looking at the OP's rigs:
- The 115BLX weighs 69lb, and uses a short-throw 15, rated at only 200W. Fart-out city with a 5-string and a loud drummer.
- The B2RE weighs 26lb, and only puts out 250w into 8 ohms. The bass control is centered way down at 40Hz. Cranking on that sucker is a bad idea in the first place, but stacking it with a 40Hz bottom slider on the graphic EQ is a recipe for disaster.
- The Backline 112 is a decent practice amp, but it's hardly G-K's "A game".
Substituting any number of newer combos for the 112, would net an improvement. Suggested candidates: TC BG250 or G-K MB212 for starters.
For the big rig, the combined weight of a fEarful 15/6 and a 500-900 watt micro from G-K or Genz would be less than the OP's cab by itself. More importantly, they would blow his current gear away in terms of loudness, low-end extension and clarity. And that is completely independent of how much time he puts into his technique, string choices and bass setup.