Just do it both ways and see if you have a preference. I'd probably start with setting the mains to small (i.e. use bass management) and start with a crossover of 80hz. Then try the speakers set to large instead, see if you have any sort of clear preference. I've seen several times recommendations to avoid the powered woofers in the DT speakers with the sub pre-outs, and if you have a good sub I'd just use the good sub. There's really no "rule", YMMV.