(half-wishing I ordered the 15" instead of the 12" extension cab)
Probably should've, based on my experience. My main rig is a couple of relatively tiny cabs, Nahas 1-12 with horn and a 1-15 with no horn. I've been using them for several years and trying other stuff out in the meantime. Still my go to rig.
The 15 is a perfect complement to the 12 and vice versa. The 15 just adds a much more immediate low end kick and reinforced the 12's punch. I've heard bigger boxes go low but they don't hit as hard in a punchy sort of way, they don't stop and start as quickly as this setup. It's a thing of beauty on a gig.
On many occassions I've gotten the "where's the rest of you amp" look, only to be followed by amazement and big grins once I soundcheck. It sounds like way more than the sum of two cabs. Adding another 12 would sound nice but adding a 15 gives me a kick I don't get otherwise.
You can get an idea of how small the rig is by the gig bags beside it.
Side view, with a Bag End S15-D. AMP BH420 amp.
