Just curious if there is any real reason you would ever want to turn the subs off? What we have found in our boat, since adding the mono amp, and 10" sub, we can run the entire system louder since all the lows are send to the sub, rather than blowing out the other speakers.
The Kicker Towers and 6.5" Alpine Marine speakers could not handle the lows at high volume. That's not an issue any longer, and we find that if we are cruising the lake at speed, and want to run the stereo at a level worth listening to, the sub has solved that issue.
Good luck, great work so far!