Re: qts too low for door speakers?
Neither of those choices is optimal. The DC160-4 looks more like a small subwoofer with the Fs of 30.3 Hz and the FR of 30 - 4,000 Hz. The 6PR-8 won't give you much bass since the Fs is 75 Hz. A sealed or ported enclosure would help extend the bass response, but in a door you won't have that. This is a non-issue if you have a subwoofer.
As for "sealing" the doors, this refers to sound deadening treatments that specifically address the usual large access holes in the inner sheet metal. The door will never truly be sealed, but it can become a decent enclosure depending on your goals.
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