Where should I place my subwoofer?

Not "wherever it fits". Placement changes a subwoofer's performance more than the model you chose, and it is the one upgrade that costs nothing.

Bass in a room is governed by room modes: standing waves set by the room's dimensions that boost some frequencies and cancel others depending on where the sub sits and where you sit. Move the sub a metre and the response at your seat can change by 10 dB at some frequencies. So the sub's position is not a detail; it is most of the result.

Corners load the room and boost output, which is useful, but they also excite every mode at once and often produce boom. Mid-wall positions trade some level for evenness. Which is right for your room is not something I can tell you from here; it depends on the room's dimensions and the seat.

The free method, and I mean this seriously: put the sub at your listening seat, on the sofa if it fits, and play bass-heavy music you know well. Then crawl around the candidate positions along the walls at roughly ear height, listening for the spot where the bass sounds tightest and most even, not just loudest. Because the sub and the seat are interchangeable acoustically, that spot is where the sub should live. Yes, you will look ridiculous for ten minutes. It works, and it is how the professionals start too.

Then set distance and level in the AVR, and check that the phase or polarity setting gives the fuller sound at the crossover.

If the best spot is domestically impossible (it very often is: behind the sofa, in front of the door, where the plant lives), that is exactly what measurement-based calibration solves. We find the best achievable position from the ones you can live with, then correct the remaining peaks with EQ. Peaks respond well to correction; nulls do not, which is why position comes before EQ.

Ask before you drill. A floor plan with the sofa marked is enough for us to suggest two or three positions to try.