Thanks, my (limited) understanding of speaker ohm loads and power (watts) is that unless you have a high powered amp, you want to stay with the same ohm speakers.
I did not do this for the front/rear door replacements, I went with 4ohms and they sound great (they are rated @ high sensitivity), but I am using more power (watts) to drive them. Typically, going from 2 to 4 ohms would cut your power output in half, meaning if the Amp was putting out 50wpc @ 2ohms, it will produce 25wpc @ 4ohms.
I have not been able to find out how the 540watts / 9 Channels of the Passport (Touring/Elite) are distributed and thus have no idea of how much power is dedicated to the Sub(?) If anyone has a guess or knows, please let us know. My guess is it is probably between 75-125 watts.
Subs are different animals, in custom setups with big or multiple subs, it's not uncommon to have them drive by 200, 500, 1000 watts or more mono amps. The problem with switching to a 4ohm (without adding an external amp) is that if it is being powered by only 75watts and that is cut in half (37.5watts) it won't be enough to adequately drive the new sub and it could sound worse!