How can I get the really low frequencies to hit harder?
 

I am new on this forum. I have done a lot of searching and reading and I am still not sure what would be best. I am trying to increase the volume of the low frequencies (not sure but I would guess around 25HZ). I listen to a verity of music so I want it to sound good for many different types of music. I listen to a lot of bass heavy electronic. It currently has plenty of volume, I just want the lows to hit harder without sacrificing sound quality.

I am currently running 2 x Alpine Type R 15s in sealed boxes @1.7CU Ft each.

I was thinking about making a ported box tuned to 22HZ to 25HZ. Do you think that would be a good starting point? I was also thinking about adding a 3rd sub instead. Any suggestions how I can increase the low end output?

loaded question. however lets look at the recommended enclosure specs. Typically tune to the woofers FS which the 15" type R's is 22hz

Minimum Sealed Box Volume 1.5 cubic feet
Optimum Sealed Box Volume 1.75 cubic feet
Maximum Sealed Box Volume 2.0 cubic feet
Minimum Ported Box Volume 2.65 cubic feet
Optimum Ported Box Volume 2.9 cubic feet
Maximum Ported Box Volume 3.15 cubic feet
Frequency Response 20 - 400 Hz
Sensitivity 87 dB

Ideally your not getting the output you require very simply because you are running a sealed box versus a ported (typically 3db difference). Sealed box having a much higher freq tuning versus designed a ported box and tuning lower from 20hz-25hz thus increasing output and giving the lower octaves more breathing room.