I’m looking at a 1500w rms system, should I get 2 skar sdr 18s or a single more expensive 18. Any help is appreciated.?

Just get a single 18 inch woofer with a high xmax. A woofer with an xmax of around 20mm will produce it down loud and low as well. But a woofer with an xmax of over 30mm will produce it much louder.

The most important thing is the type of enclosure that the woofer is mounted onto. If it's gonna be mounted onto a small sealed enclosure then it won't get down loud and low. The next important thing is how the subsonic on the amplifier is setup. If the subsonic is set above 30hz then the woofer won't get down loud and low. The subsonic is basically a high pass filter and it should be set around 30hz.

I guess all you care about is being loud?

If that s the goal then cone area is your friend. Big cheap subs will be loud, but to get LOW and loud you need the higher priced subs with big xmax.

It just all depends on what YOU want.

A single 18 at 1500 watts RMS in a good enclosure running on a high quality amplifier will be loud enough to be heard from 1 mile away. Just turn the system down when you enter your neighborhood.

Low deep frequencies of 15hz to 30hz will be produced much better by larger subs like an 18" a 21" or a HUGE 24" sub. You need a heavy 80+ pound SPL sub with a HIGH xmax to succeed this.

A decent size sub of a 10" 12" and a 15" will produce it loud and low as well but larger subs with a HIGH xmax will produce it much louder. The louder and lower you drop, more people will hear it because the lower frequencies are traveled further.