Hi Steve,
"shag" your sub...that is a new one..
The problem with mismatching the port configuration and the amplifier setting is the potential for extremely large cone movement. With a ported subwoofer, the woofer "unloads" with frequencies under the "tuning point" of the enclosure. This means the cone loses back resistance (from the enclosure) and can easily move further than it was designed to do. So an electronic subsonic(highpass) filter is usually applied near the "tuning point" of the enclosure so attenuate frequencies under this(tuning point) to protect the driver(this is the control on the amplifier marked 12hz, 16hz, 20hz, 25hz). If you set the amplifier control HIGHER than normal...it isn't a problem for the subwoofer because you are simply filtering the deepest bass a little before the "tuning point" of the subwoofer enclosure. For example, if you had all ports open and set the filter to 25hz. BUT, the opposite can lead to trouble. With all ports open the driver will begin "unloading" <20hz. That in itself presents a risk. But with the amplifier set to 16hz...instead of electrically attenuating the signal(<20hz)...you are actually applying a BOOST to that signal.
I guess you can think of the subsonic filter settings on our subwoofers as a "rev limiter" on a race car..
And with all ports open and the amp set to 16hz...instead of the rev limiter capping the engine revolutions at say 6000 RPM...when you hit 6000...the rev limiter automatically increases the engine revs to 6500. Yes, the "engine"(subwoofer driver) can handle this most of the time...but it only takes once to end up with some "metal on metal" noises(in the engine and subwoofer driver).
Tom V.
SVS