There are multiple ways to go about this, prior to me having a mic and REW. I used this method:
Take a SPL Meter, if you don't have one, down load an app on your phone.
Sit in your MLP and play frequencies starting at 20 Hz in 5 Hz increments up to 150 Hz; and write down the SPL for those frequencies. Let they SPL Level out prior to recording the SPL for the playing Freq.
After reviewing the data, you will be able to tell what frequencies you are Lower and Higher at the MLP.
You can then move the sub to another location in the room and re-do the process above.
Adding a sub to your room will typically help smooth out the response. Note, that 2 subs, up front placed equally apart, is typically NOT IDEAL.
I hope this helps!
This is only 1 of many ways to attack this issue.........GOOD LUCK!