Dunno about a microphone but I'd think a Denon mike wound be best.
I don't think speaker stands contribute that much to the overall sound but you might want to raise the center speaker
Playing with speaker placement and room acoustics is probably the best next step.
Don't even think about more expensive wires improving anything. All it improves it the bottom line of the store that sold it to you.
But, most importantly, why would you even think about kerffutzing with the insides of your amp. If it ain't broke don't fix. If it is broke, send it to a pro. There are no user serviceable parts inside.
Somehow, I seemed to miss what subwoofer you have. You DO have one, don't you?