Good advices here. More ideas:
I would say I went other way than most: lesser and lesser boxes for me. I started with 2x10, then with 1x12, then with 1x8, ended with 1x8+1x6", these two boxes I am using in the bigband with no trouble. I never had a problem achieving enough volume, as with all my setup the volume limit was always given by the feedback (I was generally using ca 350w/8ohm amps.)
The feedback problem I can solve best by careful placing of the box(es). The lighter, more operable box is better than heavier one. You want to point the speaker at your head (within reason of course), not at the body of the bass. In my experience, when I can hear my bass clearly, everyone in the band can hear my bass just alright.
I'm a bit surprised that your 2x6" is enough for bass guitar, and not for double bass. In my experience I need much more lo freqs and more amp power for bass guitar. The reason, I guess, is that the double bass frequency response is very strong in the lows (at least with 'orchestral' basses, such as mine). When you pluck E string, it starts at about 40 Hz, peaks strongly about 70-140 and then gradually fades up to 1kHz or 1.5kHz. It seems to me possible that HPF at 150 Hz might be set too low, because the cutting factor might be something like 10dB/octave, rendering the lows still very strong even when cut. Try to tweak the HPF and simultaneously try to turn bass freq down.
Last thing: I was never satisfied with my sound until I bought Sans Amp Para Driver, which tipped the 'piezo-boom' to the 'deep-buzz' side of sound image. You might want to try to focus on preamp more: the better sound into the amp, the better sound out. (Of course, my sound is still piezo, I'm using Realist pickup for my stage amp, forcing the mix guys to mic me out to the PA.)
