Re: if i'm sticking with 4x6 -- is it worth it to get a plate?
I can't speak to the Polks but, I have a set of the Boston SL80 6x8 plates in my truck and they sound great. Based on my experience, I would go with the 746.
Vehicle: '99 Ford F-150 Extended Cab / '98 Chevy Cavalier LS
Head Unit: Kenwood KDC-X995 / Kenwood KIV-BT900 w/ HD Radio
Sub: FI Audio x12 4ohm w/BP 1cu ft sealed / 2 GTO804's 1.4 cu ft @28Hz
Sub Amp: Hifonics BRZ1700.1d /
Front Door Speakers: Massive Audio CK6 / Massive Audio CK6x
Rear Door Speakers: Boston Acoustics SL80 / Massive Audio VX69
Amp: MBQuart DSC4125 / Massive Audio NX5
Wiring: All Knukonceptz
Sound Deadening: eDead 45 / AT60 / Murder mat BL
References: Retroaudioinc, davidtemple, cd6551, double07, Ssteele18