Powered subwoofers running into power amp?

Actually...plenty of powered bass bins (Mackies for example) have a "pass through" feature that will work exactly as you described. I've sent stereo signals to a powered sub equipped with SIGNAL outs from it to pro powered main speakers, and it works perfectly. It's how the pro sound world works (mostly). You can take these same outs to any pro power amp hooked up to passive speakers (which I've also done about 462 times), and your level will be cleanly controlled by the mixer, unless you have no clue what you're doing and crank everything to the point that it explodes and causes your girlfriend to dump you for an ear specialist. The Mackie site has all this spelled out.

El redundo amundo. "Loading" of the low impedance balanced signal from a pro audio mixer to the balanced inputs of pro amps in the real world is nearly non-existant, and nobody who mixes shows (like me, for decades) worries about this even when using 100 foot cable snakes driving multiple amps.

It's hard to imagine he found a mixer that doesn't have balanced (either XLR or tip/ring/sleeve) outs...but I will conceed it's possible, amigo. That said, if he's that unsophisticated I doubt he understands any convoluted explanation regarding capacitance.