Sorry I didn't respond earlier. I've tried crossover freqs at 80, 100, 40 and now 50Hz. The last one gives me the best results so far. The slope for the sub is at 24dB so it doesn't interfere much with the Quads above the crossover frequency. The difference between 40 and 50 is subtle but useful for some low instruments. If you check my system page, you'll see that the position of the sub is somewhat unusual but works much better in my room; perhaps that's also why 50Hz works well.