I'd thoroughly check (and swap) the cables. If that doens't lead anywhere...
It sounds symptomatic of a cracked trace, cold solder or the like. If you need to have your subwoofer amp's volume set very high for the problem to suddenly cure itself, it's likely a bad connection (trace or solder) in the output stage. A wild guess having not seen or tested it myself :-), I'd say it's a cracked trace (since knocking gently seems to cause it to change). But if replugging the RCA fixes it, I'd be wary of parts that are stressed when you plug in the RCA. Having not torn apart an EP175, I don't know what those might be other than the jacks themselves.
I had a very similar problem in one of my Polk RT2000p's. The subwoofer amp wouldn't kick in until extremely loud volumes at both the input (speaker level) and the output (volume on the subwoofer amp). It was due to a cracked trace, I suspect caused by damage during shipping (since it arrived with a chip in the base plate. Once it came on, it worked fine all the way down to very low levels, it just took loads of potential for it to come on. Any evidence of rough treatment during shipping?
