Did you buy the speakers new, or used? I have seen speakers that had been serviced and a driver's polarity was reversed. One was in a recording studio- the owner asked me to bring the RTA from the car audio shop where I worked and as soon as I heard the sound, it was apparent (to me) that they had a phase problem and the RTA confirmed it by showing a deep V in the response in the crossover region. When I asked if any drivers had been replaced, he told me the tweeter on one monitor was new, but he hadn't been the one who did it. Once the wires were reversed, all was good.
Turn the balance control to one side and listen- if it improves the sound, find out why. On an AVR, you would obviously do this by adjusting the speaker level in the menu. If the bass improves, it would point to a reversed woofer OR possibly the speaker wires.
I find it odd that you're blaming amplifiers and then, describe the speakers' sound as 'thin'. That and the reflected sound isn't a good combination if you like a warmer sound.
If you're using a sub, you're not hearing any results of an amplifier controlling anything, for better or worse.