Newbie trying to improve dialogue

Hi, please let me preface this by saying I am totally clueless so please forgive any stupid questions.

I wanted to improve the sound from my Panasonic tv and after being unimpressed with a couple of soundbars I decided to try a home theatre setup. The main objective is to achieve balanced sound with clear dialogue, I'm sick of using headphones all the time so the upstairs neighbours can't hear my tv.

I already had a Yamaha amp (RXV365) which has been in the cupboard for years - although I've always been suspicious that it might be faulty.

Bought Q Acoustics 3010 Bookshelf Speakers for my front left and front right output. And as a starter, the sound was promising. To build on this I added a Q Acoustics Q7000i Centre Speaker (from eBay). Simply put, my reasoning was the front left/right speakers will control the atmospheric sounds, whereas the centre would handle the dialogue (is it that simple?). Well the sound from the centre is really muffled. As a test I hooked the centre speaker to the front right stereo connection and the sound was pretty good so don't think there's a fault with the speaker.

At this point have I already said something stupid - was I naive to think these 3 speakers would achieve the effect I was looking for? Do I need more speakers/sub-woofer to draw bass away from the centre?

I'm thinking the amp may be faulty, or an upgrade might help - but before I go that route it would be helpful to have some comments on what I've tried so far.

Thanks in advance.