And they're phase-correct, right?

Do you mean : are the pos and neg terminals correctly wired ?

They should be. But mistakes can happen I guess ! Is there an easy way to test without taking the subs out of the enclosure and looking at the wiring ? Multimeter test somehow ? It breaks my back every time I have to fetch these out of my car !

The output at <40Hz is fierce : real stomach churning stuff, so I has assumed phasing to be ok. The amp is single channel, so I made an internal wire to connect everthing up in parallel. By this I mean :

the amp neg terminal is connected to every voice coil neg terminal
the amp pos terminal is connected to every voice coil pos terminal

Just wanted to be clear on that last point ...