The advice provided in the previous responses, while both well intentioned and appropriate for situations involving many subs and many amps, should absolutely NOT be followed in this case.
First, it appears that your sub does not provide speaker level inputs. The "hi-level" setting of its input level switch corresponds to the line levels of signals that are typically provided by pro audio equipment, not to speaker level signals.
Second, even if the sub did accept speaker level inputs, the negative input terminal it would provide should not be connected to the negative output terminal of your amp, since your amp has balanced outputs. From the manual for the INT-150:
Also on the rear panel are the output connections for the loudspeakers. On stereo amplifiers there is one pair per channel, and on mono amplifiers we provide two pairs wired in parallel to facilitate “bi-wiring” a loudspeaker. As you are probably familiar, the red one is positive polarity, and the black one is negative.
Please note that, being a balanced output device, both of these terminals are electrically live – neither is grounded and neither should not be treated as ground. This is particularly important if you are using the output of the amp to also drive a subwoofer amplifier or other active component. If there is any question, please contact the manufacturers of the products involved – both they and we will be happy to advise you.
If you happen to need a clean signal ground connection for any reason, we provide a dedicated 5 way binding post just for that on the rear of the amplifier.
The negative input terminal of a sub which accepts speaker level inputs should be connected to the aforementioned 5-way binding post.
As to what the problem may be, after looking over the manuals for the sub and the amp I have no idea, assuming (as it appears) that the sub’s controls have been properly set.
Good luck. Regards,
-- Al
