if your sub has a floating ground (i doubt it) then why did the manufacturer put it there in the first place. Do not remove it and do not break the earth pin from the AC connector.
Since the use of a cheater plug apparently eliminates a ground loop issue, AC safety ground is presumably connected within the sub.
On another note, I wonder if the manufacturer’s ok about defeating the AC safety ground was put in writing, e.g. in an email, or if it was stated verbally. Usually a company will avoid making statements that would create even the slightest risk of a lawsuit.
And on another note as well, given that this is a sub its housing is presumably non-conductive, which would reduce the risk of defeating the safety ground connection compared to doing so with a component having a metallic enclosure. However some subs have metallic heatsinks on the rear, and all of them have screws, connectors, etc., which conceivably could become electrified with 120 volts in the event of an internal fault, perhaps one that is eventually induced by the vibration that occurs within a sub coupled with a manufacturing flaw. And in addition to creating a safety hazard such a fault could potentially damage the rest of the system, depending on how safety ground and circuit ground are interconnected within the sub. The purpose of the safety ground, of course, being to cause the circuit breaker in the electrical panel to trip in the event of such an occurrence.
Regards,
-- Al