I own an NV212T which is my #1 go-to cab. I find it to be plenty loud by itself for most any situation I ever find myself in, so unless your band has multiple loud, low-mid heavy guitars, other instruments that are competing in your frequency range, or it is just frigging loud in general, I'm surprised you're not getting enough volume. Since you describe the room you're in as being boomy, maybe you should make a judgement on the sound of the amp after playing in a more sympathetic acoustical environment. Sometimes the room is everything.
Power handling on the NV212 is rated at 400w, which is what the Demeter is putting out into 8 ohms, so I don't think being underpowered is the issue - although since most manufacturers conservatively rate power handling specs on their cabinets, I'm sure the NV212 can easily take a bit more than 400W. When you say you're only using half the power of the Demeter, that may be true in terms of wattage, but you might experience the difference more in increased headroom than perceived volume.
Although the tone controls on the Demeter amps are a bit subtle, I doubt if cutting mids is helping your sound cut through the mix. In my experience increasing mids, rather than decreasing them is the solution to that. I would describe the Demeter amps as being a bit on the "polite" side, so maybe a more aggressive sounding amp would serve you better.
Having experimented a lot with using multiple cabinets, I've found that it is hard to get different cabs to play well together. There can often be problems with phase coherence and differences in efficiency that result in unbalanced volume between cabinets (especially if you don't have a true stereo amp). I've found that multiple cabs nevert sound as focused as a single cab, to my ears anyway, so in your case if you feel like you need to move more air I would instead look to a larger single cabinet solution. I'm sure others will disagree, maybe even with everything I've said here!
