Best way to add two speakers to a 4 channel amplifier that only has one available ch?

Riskay

I'm adding two speakers to my car and would like to know the best way to wire them.

First off, my current set up consists of:
One pair of Polk Audio db651 6.5" door speakers/one per channel
One JL Audio 6w3v3-4 6.5" subwoofer/not bridged using one channel
Polk Audio D4000.4 4 channel amplifier

Currently, each speaker is running on their own channel with one channel empty. I'm trying to figure out what's the best way to add another pair of the Polk Audio db651. Should I wire them in series or in parallel? Are there any other changes I should consider making to my setup?

I've had the extra pair lying around and figured since I have a free channel I would try and install them. This is for a Miata so I wanted marine grade speakers and also explains the limitation on space.

jtapoling

I think you would have to run them in series because your amp would not likely step down to handle the ohm rating running them in parallel would produce.

hewlew1

They are 4 ohm drivers so unless your amp can handle a 2 ohm load they would have to be wired in series producing an 8 ohm load on the amp.

20hz

old trick run 8 ohm speakers in the back paralled of the front speakers ( off the polk db651's ) .
I think the db651 are 2 way if you run them in series it will cross all the speakers over at what ever the tweeter is at ?4khz?
miata have real deep doors but a second set of speakers will be absorbed by the seats , improve your sound with some piler tweeters .