Flat upper extension on a sub? Hmmm... I have always found the problem to be with directionality as the frequency gets much north of 50 - 100 Hz. In controlled test years ago it was easy to localize a sub at around 100 - 120 Hz, almost impossible at 50 Hz, in between a gray area. With satellites crossing over around 80 - 100 Hz, there was enough energy in the upper bass to me that I felt dual subs were needed. My main speakers go low enough that isn't an issue, but I like the idea of duals for various other reasons. YMMV.
Given the slight power difference (less than a nightlight), auto-on only matters to me if it's needed to control turn-on transients (thumps). Not a problem I have seen in the past decade or two, at least for anything I have tried. My old vintage (ARC SP-3) tube preamp does need some warm-up time, however, if you don't want to lose an amp, and/or maybe a speaker. Trust me on this...
As it stands, the limiter sounds like a good idea to me. I doubt it will have any audible impact in the subwoofer range (higher, yes), and it sounds like an idea that will save a lot of speakers and EARS! Hearing damage is not a good thing.
I would be glad to post what I hear (which of course may not be anything like what you
hear) from an A-B with the limiter on and off once my new subs arrive. For equipment I am using Magnepan MG-IIIa's driven by an emotiva XPA-3 for my mains, from a Sony ES3400 receiver, and with an Infinity center (driven by the emotiva) and Mirage surrounds (driven by the Sony). However, by this weekend I should have the new subs plus a new center and surrounds from Magnepan so it will be quite an experience. My first major system upgrades in years.
New toys! - Don
p.s. Not sure how ground loops snuck into Brian's reply to me. I have many years' experience doing live sound and sound installs though little recently (not my day job) and ground loops can be terrible things. Despite the "quick fix" they often provide, a cheater is my last resort for many reasons, not the least of which is that they can be dangerous. Besides the obvious shock potential, my neck of the woods is very dry (CO front range) and static is a big problem. I want the ESD going to a good ground, not inside my components...