Usually it wouldn't matter if the speakers "respond" to 35Hz. The idea is to highpass the speakers a little higher, reducing excursion demand on the midbass woofer, and thus reducing distortion as well. That's the usual way to go about this. If, however, you have giant midbasses in your speakers, then it might make sense to transition to subwoofers as low as 30-40Hz. It's difficult to imagine that for any loud output level, the subwoofer wouldn't be cleaner than the midbass below about 35Hz, however I don't tend to model smallish (or normal / average) bass setups and speakers in small rooms. So perhaps things are a bit different when the subbass has just 1x 12" driver to rely upon. In those setups, I tend not to use a subwoofer at all and just rely on powered monitors (of course the sub would help, but space is always at a premium).
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