theoretically, could i use a decimeter and a wide variety of sine wave tones to take measurements on the amplitudes of certain frequencies, then use an equalizer to correct the frequencies so they are at relatively close amplitudes to reduce the curve in the frequency response?
Sort of.
Depending on the SPL meter, the accuracy at high frequencies could be seriously deficient. And even if not, without a calibrated mic the results will still be a bit off, as every microphone will have its own inaccuracies.
Now a bit of inaccuracy may not be a huge problem, but you
will need to measure the speakers in a way to get the data you want, and you will need to know how to interpret the data.
If all you want to use is an SPL meter and tones, I recommend using band limited pink noise, or warble tones instead of sine waves, and put the meter very close to each driver, say an inch or so. That will give you a (very) rough idea of what the on-axis response of each driver is (but you will also need to compensate for the measured level difference between each driver). But even with this information, you still have a very hazy picture of the acoustical properties of the loudspeaker in question, as the off-axis response and how the individual drivers sum would still be unknown.
It is important to know that the reason for the close mic placement is that unless you are using tools that allow for a windowed measurement, you will be measuring speaker + room. Not the information you want for what you want to do. Depending on how much you want to get into this,
this book could be very useful.
