Suggested Improvements for Set Up?

Looking for advice to improve my current set up, which is as follows:

Polk LSIM 707 (Left and Right)

Polk LSIM 706C (center)

Polk DSW MicroPro 3000 (sub)

Yamaha RX-A3020

Apple TV

Panasonic VT50

What would I get greater improvement from a DAC or separate power amp?

Set up:

Fronts are currently approx. 8 feet apart with 22 degree toe in

Sub is located between left and media cabinet

Center is on top of media cabinet

Any suggestions...