If it sounds good through headphones, you can rule out dead spots as the problem. Unless you have a parametric set for a deep & narrow cut at ~62 Hz or 124 Hz, you can also pretty much rule out your signal chain to the board. (Other processing and effects can make your bass seem to disappear in the room, but they usually don't do so only at octaves of one note.)
What sort of music are you playing, and can it be played quieter for rehearsals? That is, can the band play a 12' x 15' room at a volume that doesn't require bass (and any other instruments) to be run through the PA as well as through your 210? If you can, cut volume, and bring instrument cabs up closer to the players' ears. Bringing volume down will reduce the smearing effects of room reflections, and having your bass come just from your amp will spare you the comb filtering you'll get from running the bass from three points in the room (bass cab + two PA speakers).
If you're playing doom or somesuch, or if the band's enjoyment depends on making the room vibrate at the molecular level, then at least stack the two PA cabs and put your bass amp with them to cut down comb filtering. (Also, put a high pass filter across all the channels except bass and kick, to cut down the unproductive low end spill that will interfere with your sonic territory.) And wear earplugs so you don't suffer tinnitus for decades.
But if you want to hear what's everyone's actually playing in a tiny rehearsal space, it's much better to turn way down. If that's not an option, the second-place solution is to use closed ear headphones or IEMs. Unlike running through the PA, 'phones will reduce any crazy SPL from a maniac drummer or guitarist/volume junkies, letting *you* hear everything clearly while monitoring at a lower level. Phones will also help a lot in letting you work on vocal harmonies, spot and fix clams, and other rehearsal work for which you need critical listening.
