Actually, I use the Xover in the preamp mode, and then use a SET (a 45 SET) to drive the "single driver". Sounds quite nice to me. I don't personally believe the opamps make a difference, (any more so than the myriad other phase shifts caused by passive compknents, which I donlt have, box, room reflections, etc). I have experimented with the same driver ( in the same box) in single driver mode, single driver with BSC circuit mode, "single driver" with super tweeter and BSC mode and finally, "single driver" with super tweeter, sub & active Xover mode. THe single driver with super tweeter, sub & active crossover sounds by far the cleanest and the most involving.
But this is just my opinion, and may be different for different setups.
-akhilesh