Changing the shape of the room will likely cause reflected soundwaves to interfere at specific frequencies which results in unwanted volume spikes at those frequencies. It doesn't result in an echo, it's more of an uneven volume issue where you can never balance the high/mid/low properly because certain notes will always resonate louder than other notes. In a typical 10-20 foot wide room, the resonance peaks caused by reflected sound usually occur between 200 and 500Hz so it's going to be in the bass/tenor range of sounds where you hear that. You should be able to sweep a sine wave tone from about 80-8000Hz without any obvious differences in volume. No speaker is truly flat, but big dips or spikes in the volume are often the room's fault and not the speakers.http://www.szynalski.com/tone-generator/
Echoing is almost certainly a DSP issue with the receiver. Does your receiver have a stereo output that you can plug some headphones into, to check if it's also echoing on completely different speakers without any weirdness caused by reflected sound bouncing around in a bay window?