so i think i have this right: SPL = x dB using 1W 1M away from speaker... dB actually = dBm? EDIT: Oh wait, dBm wouldn't make sense would it? It's a comparison of the actual amout of physical pressure in the room vs. the additional pressure from the cab, right? Anyway, what I don't understand is how this relates to different sized cabs. So you've got a 2x10 @ 100 dB and you've got a 4x10 @ 100 dB.... Does this mean the 410 is less efficient since it requires all 4 speakers to create the same SPL?