Just to see if I got this straight. Is it the higher the frequency ( > 120mhz) the louder but the lower the frequency( < 80mhz) the boomier?

Um no. Hearable sound ranges from 20 Hz (bass) to 20 kHz (treble). Bass frequencies are actually between 20-125 Hz approximately. The very low frequency will be the deep bass heard in rap music, whereas the higher frequency bass will be tight like that in trance or techno music.

How is that different than what I said?

Neither frequency is "louder" than the other. Frequency just has to do with how high or low the tone is

If your system consists of 500 subs and 1 tweeter then the frequencies <120 would be louder.

But yes the lower is boomier. I've never heard a boomy sounding cymbal. Loudness will depend on your particular system.

Frequency does not correlate like that with loudness or boominess.

Your mhz frequency range refers to approximately AM and shortwave radio frequencies. Loudness is the volume, boominess is the uneven volume at different frequencies.

SO is frequency the tone, and DB the loudness of the tone?

yes, that's why some radios have volume measurements in +/- dB.

Frequency is the tone, or what note it is. For any given frequency, if you double the frequency you've just gone up one octave. So 1000Hz is one octave below 2000Hz, and 500Hz is one octave below 1000Hz. It's like if you play guitar, you know that if you play the 12th fret it cuts the string's length in half and plays it an octave higher then when it's open.

So in theory, the higher you set the crossover for your sub, the greater range of notes it will be able to play. However, depending on your amp/sub/tastes/equipment, going too high or too low may sound bad.

None are necessarily louder, but a higher frequency has a higher pitch to it, and a lower frequency is your "bass" range.

BTW it is Hz not MHz. MHz is millions of Hz.

The lower the frequency, the more power it will take to make the sound. So, practically speaking, if you cut your speakers off very low, it will take more power (and thus, will crap out at a lower volume) than if you cut them off very high.

However, given that you are (I hope) running the signal to your sub through a lowpass filter, not a highpass filter (i.e. generally speaking, everything below the cutoff is played, not above), this doesn't really apply. The point of the cutoff in this case is to help you get a flatter/more accurate sound when balancing your sub and speaker output. Using the cutoff to change what the sound is like, is like using one of those stupid "bass boost" buttons. The goal when designing a system is to get true, accurate sound, which basically means all the frequencies play at the level they are supposed to and aren't boosted higher or lower than they should be.