I have an HSU sub, and although it does not exhibit this problem I noticed that in the manual it say's to just leave the sub turned on all of the time if the auto switching circuit does not operate, and that this will not cause any damage. I guess different receivers have different gain levels on the sub output.
Another thing to try would be to crank up the sub level in your speaker level setup, and reduce the power amp gain on the sub amp accordingly. In theory this will give you a higher 'low level' input for the same overall SPL and may cause the trigegr to operate again.
