Regardless of whether it's a PA5 or a PA5II, for a subwoofer it's really like throwing pearls before swine and wasting money.
It's the wrong application for this amplifier anyway.
An amplifier designed for 2 ohms, such as the well-tested AIYIMA A70 MONO, is significantly cheaper and has a much higher output when needed.
And if that's not clear, too little power is a problem, too much is not.
You should definitely film that for the forum and for YouTube.
With many amplifiers, especially Class D, the loudspeaker minus connections must under no circumstances have contact with each other or with ground. This can lead to short circuits and smoke signals.