How to do room acoustics correction the easy way.

The question begs an "and" in place of the "or."

If your garage is bare walls then your midrange is being "smeared" by slap echo. Stand near one wall, clap hands, hear the flutter as the echo bounces back and forth. That's slap or flutter echo.  

There's a whole range of different things to attend to in a room like this. Trying to correct response alone like with a DAC is actually the last thing to try, if ever. Wouldn't touch that to save my life. Too serious about sound quality.  

That's why you read the book. To learn. You have so much to learn you can't believe it. Owens Corning 703 is a standard acoustic panel you can find at many hardware stores. Its an absorber. These are great for stopping reflections like flutter echo but it does not take a lot before the room starts to sound too dead. Diffusers are panels that reflect but by sending the sound off in many different directions, shifting timing, etc. This also breaks up echo but in a way that preserves spaciousness without making the room dead like absorbers do.

You want this balance and there's really only two ways to get it. One is pay through the nose for some guy to tell you what to do and sell you a lot of very expensive panels. Great results, but you could buy a car and a nice one too for what they want. The other way is get a few panels (much better and easier to work with than your blankets) and start experimenting. 

That's what I did. Actual plain OC703 panels on the walls. https://theanalogdept.com/c_miller.htm Sounded good but too dead. More recently looks like this https://systems.audiogon.com/systems/8367 Could have paid someone for this. But I know what that costs, and what a crap shoot it can be. This way takes longer, but costs a lot less, and I actually learned a lot about what's going on in the process. 

It all started with the book. Read the book. Seriously. Study it. Learn it. Live it. You can thank me later.

Thanks to all...bought the Harley book and ordered the acoustic panels.

Update....I purchased 6 Corning 703 panels, 4’ X 2’ X 2”.   I placed one behind each speaker and 2 in each front corner placed on top of each other for an 8”height.   I was shocked at the difference in bass resolution.  The sound stage opened up voices became more precise.  I just was not prepared for how much improvement there was.  Thanks all for the guidance and advice.  What an amazing bang for the buck.  I’ve been in front of the hifi for over an hour listening to music I’ve heard 100’s of times and it does sound better!.. Grateful this is a room my wife doesn’t have decor authority over!