Lee, there may be some problem with terminology here. You say that it's an "external" sub amp with "High Pass" outputs to the sub. If you mean that, rather than an internal amp within the sub enclosure, with high pass outputs(which remove bass and pass higher frequencies), it couldn't be connected effectively to the sub, since the bass frequencies would be missing.

If you can, clarify what the form of the present sub amplification actually is.

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Enjoy the music, not the equipment.