Okay, picking up again; yes some subs have a filter on their high-level speaker output terminals which do a high pass to the mains somewhere around 100Hz. Although some Velodynes have this, it isn't mentioned in the VX-10 brochure and the owners manual apparently isn't available online(ask Josh?). The Hsu sub doesn't have the high pass; the statement on the Hsu site that it does is incorrect.

You ask about the RCA input, but unless the NX10 has a sub output you can't use it; you'd have to use the speaker-level connection. The sub is of course mono, but when you use the left and right speaker level outputs to the mains that preserves the stereo information.

I also use headphones(AKG 501s). Don't expect to ever get a headphone type of clarity from any speaker playing in a reverberent room, regardless of design or price. It's the room.

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Enjoy the music, not the equipment.