So. Come to find out, the SB-1000 has an 80 Hz high-pass crossover on the RCA outs!
I woke up in the middle of the night and remembers that the sub has RCA stereo line level ins and outs, and I thought to myself "if it was me, I'd put a crossover on those outputs." And sure enough, it's there. It seems to be a steeper crossover than my AVR (the AVR 80 Hz crossover will have some signal still going to the towers at 20 Hz, the crossover in the sub has virtually none), and it hasn't been as easy to integrate, but it's good to hear what the stereo setup with the towers with bass management sounds through the Emotiva A-300.