How Do I Adjust A Subwoofer For Best Sound?

Those 6.5's have impressive specs-


Hard to find 40 Hz response in something that small.
However, the magnet looks tiny.

I've found "sound guy's intuition" judging of speaker by the magnet size to be basically true. Bigger magnet = better speaker.
The same can be said for amplifier power ratings.

Look at the phisical size (and weight) of that head unit...and consider what all else it has to do besides amplify.
...and it's rated at 4 channels of 40 watts or 160 watts total?
Then look at that seperate amp, it's size and weight (and cooling fins)...but "only" rated at 30 watts by 2 channels or 60 watts total.
Something doesn't add up.