I'm setting up a new TV in my basement.
My current speakers are a collection I've picked up from various times in the past and outlined below. I can live with the Surround, Center & Subs, but I need to get some mains that are in the wall to please my wife. My budget would be $300 or less for a pair of Left & Right Main channels.

Here's my setup:

The room is about 13 x 21 x 8.5' ceilings, carpet floors and my new TV is going on the long wall. I know this isn't optimal but this is existing construction with a fireplace on one short wall and a door on the other.

The wall with the TV is studs but has a 3" gap between the back of the studs and the concrete. So the entire wall has one big gap behind it.

The speakers will be powered by an Onkyo TX-SR707 receiver.
12" Infinity Powered Sub

Polk Center Channel - probably cs250s:
I think it's this one based on photo and close dimensions.

Bose Acoustimas Surround (have the 3-double cubes and base)

Current L/R Main:
Polk M5 - probably 12-15 years old so I'm not sure if they degrade.

So the burning question, what are my best options for in-wall speakers? I want something with a paintable grille to totally hide them. I'm good with construction, wiring & drywalling and the gap behind the wall will make wire routing no problem.... I think from drywall face to concrete is about 6-7 inches total.

I've read some reviews about in-wall speakers that have said the enclosed space between the studs on a traditional wall creates the speaker cavity. Obviously that won't work for me.

Also, one more question on the surrounds.... with this receiver, would I need to run the Bose Satalites throught the bose sub or should I just go straight from the receiver and shelf the sub?

Thanks for any input!