I just returned a GK 115 NEO III as I didn't care for the tone much (I WAS planning on running 2-115's). Some settings seemed to bloom, while others caught a harshness that was quite unpleasant. Maybe the cab was just too 'modern' ported for me.
The BX500 didn't respond poorly with an Acoustic B410, but you could just tell that the cab was straining a bit to stay together, and I surely did not look forward to stacking 2 of them together. Maybe the 4ohm Acoustic 810 would have been the ticket, but I'm so sure.
The 210 cab that Carvin puts the BX500 into (BX510 combo) also seems to not appreciate the head that drives it. I've found certain frequencies that the cab just finds very sour (A# high on the G string, for example) and it will vibrate with a tone that is very unpleasant. And putting a Carvin 115 underneath doesn't look very aesthetic in the least (much less now running mis-matched driver sizes in a less than optimal setting).
I broke down and ordered the BRX10.4 NEO from Carvin - I am assuming this to be a mating of heavenly sweet tones

There has got to be THE cab for this BX500 and one can only assume Carvin makes it, so... I'm giving it a go.
I'm finding that the 3 tones I'm after (smooth and bottom-heavy old-school, tube-driven gurgly break-up, and Tube-Screamer'ish clanky) a tad difficult to all real in... but at least I'm having fun on the journey.

I'd take a jazzy fat and funky tone in there too, if I can squeeze it in.
In case of any potential HELP:
- Bottom heavy, old-school: VT Bass
- Tube-driven break-up: BDDI
- Tube Screamer'ish: relying on the BX500's EQ
Tried, but no luck:
- Badder Bass Monkey
- EHX Big Bass Muff
- DOD Bass Grunge (NOT the Digitech version)
- Ibanez PD7
(still have all and will sell some day)
Of course - I could go with Ibanez's new Tube Screamer for Bass and if someone could give me FAT settings for the BX500 EQ, I could go all Jazzy-Funky fat and squishy... then I'd have all 4 tones I'm in love with. Everything from classic rock to modern heavy to 70's funk to bluesy soul
(still looking...)