Sub placement(s) behind the listening position are often justified, in addition to closer flanking subs. Naturally, the amplitude and low-pass frequency of the more distant subs will need to be adjusted so they cannot be detected as separate sound sources.I find that subs placed closer to the mains can be low-passed higher and can also have higher amplitude. Those are what I call flanking subs because of their position in relation to the mains. With higher low-pass, they tend to smooth the upper modal region.
The more distant subs are necessarily low-passed at a lower frequency. They tend to smooth the lower modal range.