AVS Special Member
Join Date: Dec 2007
Location: Southern California
Mentioned: 3 Post(s)
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It is rather odd to not have speaker-level inputs on a subwoofer.
A Polk PSW505 can be had for around $200, and has both line-level inputs and speaker-level inputs, as well as a gain control and Frequency limit adjustment. This is normal for a subwoofer.
It sounds like your subwoofers do not have the normal configuration, so you may need to upgrade there.