Room adjustments for optimum audio

Hi.
I have a basic understanding of room acoustics (Reflection, absorbtion, standing waves etc) and am seeking a source for more information.

Our HT is a converted 20x24x8 garage.

Sony projector
Onkyo 5:1 processor/amp combo
Koss (I think) front speakers, JBL surround, Home made center speaker.
Two home made subs driven by an older Yamaha natural sound amp.

The cement floor is covered by a thick,dense carpet ontop of an oversize padding. A large sectional fabric couch and I've done some basic wall treatments (bookshelves etc).

Front speakers are placed half way up the wall L/R of the 10x6 screen, surrounds near the ceiling in the back, and I've used the older Radio Shack SPL to place the subs.

(I temporarily put the sub in the main listening position and roamed the SPL around the floor at sub height while playing a 100hz sine wave and noted the highest reading)

Standing waves are terrible, but not sure where to find information about what to do about it. Can anyone suggest a reference to basic DIY room treatment for someone with limited knowledge?

For example:
* I understand bass traps are used to eliminate low Hz energy, especially in room corners. Why eliminate the low Hz?
* I just learned here at Hometheatershack that a goal for Sub EQ is flat. I always just thought max output at lowest Hz was desirable.
Now I'm curious to know if a flat response is desirable for the entire system.

Anywhere I can get basic knowledge such as that? :dizzy:

Thanks in advance
-Steve