Standard 2x4s will hold wood screws just fine. Use a screw with the largest gauge that will still fit through the bracket, and make sure it's at least 1.5" long so you have more threads in the wood.

But instead of mounting the 2x4 on the outside of the wall, since you have access to the stud area behind the wall,why don't you put lengths of 2x4s between the studs behind the wall -- either toenail them into place or double up the studs and secure the horizontal pieces to the new vertical pieces -- and then screw into the new horizontal 2x4s from the front?

I only have access to the backside of the wall (without removing drywall) from about the bottom of the TV down. Up from there is the floor for the next level up (tri-level), and the inside of a closet wall.

I think 1x4's would look better than 2x4's when mounted on the wall where you would see them, although painted to match.

Also, the experience I had with the M2 mounted directly on the wall told me something. (It was screwed into a stud). It puts a dent into the wall over time at the bottom of the bracket. They supply a plastic piece to put there that should help with this problem, but I'm not sure if that would work well. The M22's are much heaver than the M2, so mounting them directory to the wall without the wood backing might just end up poking holes in the wall at the bottom of the bracket due to the weight of the speaker.

As you can see from this picture. The hole is where the VP160 in-cabinet speaker goes. There is no further room going up. But for the 2 speakers that are on the bottom. I can add extra wood here to give them something more to screw into.

Your just proud of your new duct!

I only see a duct; check your posted picture cat! i do see evidence of water running down from the floor above though; at least i hope those are water marks!

Those are old water marks. Hard to determine how long ago. I had a new roof put on 2 years ago, so I'm not too worried.

You don't see part of the hole for the VP160 to the rightmost part of the picture? I was just showing this again to prove there's no way for me to put wood on this side of the wall, from here, for the topmost speakers.

Here's all I got accomplished this weekend. 2 photos of the new curtains for the window on the front of the house and the left side of the seating position for my small living room HT. Pictures are dark, but it's late evening. I like how it blocks out 100% of the light, except for a small amount along the top on the wall when it's sunny outside. I have another window to do at the back of the seating position, where right now, there's a green blanket thrown up with the drapes to keep out the light.

It cost $400 to do this window, which included a double rod, and 2 sets of 4 panels of curtains. Chocolate brown blackout out curtains are behind this more decorative inner set.

Here's some (hopefully) better pictures.

I'm sure it's just the lighting, but it makes me think of the Necronomicon Ex Mortis from the Evil Dead series.

I use to use 2 sets of sheets from Walmart. Total cost 25$, this included the thumb tacks to install them......