That's easy, I don't.
Since I got the Mac amp I simply no longer feel the need to run the subs with the Nolas.
However when I did, I ran them from the preamp level RCAs. The problem with running the subs ona speaker level input is that they sub then has to take on the "flavor" of your power amp. And that flavor may or may not blend in well with the way the speaker was voice, which I would assume was done with the sub's internal amp. Of course, as always, that idea is a good starting point, but a final decision needs to be made based on listening to the sub connected both ways to your own equipment in your own room.
The other thing that always worried me about using speaker level connections to drive a sub is that in doing so aren't you essentially putting the crossover in the sub in parallel with the crossover in your speakers. And if the amp's output impedance is part of the crossover circuitry, can't changing that impedance by paralleling the amp with the sub change the performance characteristics of the speakers crossover?
HAL and any of our other speaker design gurus, any thoughts on this?