Sonny,
Sorry for the delayed response. I have been so busy lately. The simple answer to your question is yes, you are getting a boost where modes combine.
A simple test that one can do to understand what I am talking about is to play test tones lower than say 200 Hz and walk around your room while the tone is playing. You will notice that in some spots it is louder and at others it gets softer. Room size and the ratios will help lend to a lessening of this effect, but will never replace the need to adequately trap the lower frequencies within your space. I have been able to measure the effects of placing bass trap in the corners of my room and the effect it had on the room as a whole. As a result the low frequency tone was more even throughout the room.
To be honest, I too am a “hillbilly” when it comes to acoustics. All I have learned has been the result of my reading Alton Everest’s book
http://www.amazon.com/gp/product/B00...lance&n=551440 . The link is a downloadable version for $22.
If you would like, I can try to elaborate more on these topics but it is all covered in the book.
Chris