From my understanding, one sub works well between speakers. 2 subs work well in the corners.
I tried placing a sub in the corner of the room and the result was terrible. The bass sounded uneven and smeared. Much better results when it was placed in between the speakers.
The key to the integration between the sub and main speakers is to have a seamless and even bass response. In my system, the impact of the sub is not earth shattering and rather subtle, but the improvement is an important one. There’s a slight lift and punch in the low bass and midbass which add to the musical enjoyment.
The speakers that are used will also play a part. With some speakers, the sub doesn’t blend well in the system. I only managed to achieve a good integration with select speakers.