I checked out the link you had - now that looks like a useful piece of software!

Thanks! I guess it seems to do what the other cannot do, which is measure and detect room modes and help with the acoustics (which is of course a large part of my headache right now)
Either way, I used the Test Tone Generator software last night. I didn't get around to checking the rolloff frequency of each of my speakers individually, but I did check the center along all five frequency bands represented with the RX EQ.
Without having to plot anything, using a logarithmic sine wave function at each of the specified frequencies as follows: 100Hz, 300Hz, 1KHz, 3KHz, and 10kHz - one thing immediately stood out and totally confirmed my suspicions that something was amiss around the 300Hz range. The SPL meter nearly jumped two ranges when I set the generator for 300Hz and turned it on. Oh, I also disconnected every other speaker but the center channel, and turned the sub off, so only the center was providing sound.
Every other frequency band I tested, registered at fairly equivalent levels on the meter, save for the 10kHz, which was also a bit high. So, after adjusting the EQ to even out the sound level in all frequencies, I hooked the rest of the system back up, turned on a normal audio source (which in this case was my digital cable box), and boy - I have to say, what a difference!
It really cleaned up the coloration of the center quite a bit, but it's still not perfect (in the world of audiophilism, is there any such thing as perfect?).
My next step was going to be to measure and plot each speaker's lower rolloff frequency to the -3dB point, take the highest measurement and use that to set my RX crossover setting once and for all.
Once that is done, all that's left to worry about is room acoustics and placement, knowing that I have my settings dialed in to their optimum points.
Granted, I'm by no means an audio engineer - just a technically inclined person with a newfound thrill and obsession.

Do you think this is a logical way to go about it?
I would really like to utilize this program you showed me - that looks like a heck of a lot of fun!