Front facing subs like that are fine close to the wall since the air movement and sound is coming out of the other side.

The reason you don't want many tower speakers close to the wall is because they have ports on the back, and there is some level of audio resonating from/through the cabinet itself, but with a sub, while that still happens, the lower frequencies aren't impacted the same way due to how long the wavelengths are.

Now, close to a side wall (but still facing forward) will definitely change the subwoofer output into the room a little, and a corner is just nasty. Yes, it will "excite more room modes" but it usually comes across as boomy and muddy.

Farewell - June 4, 2020