It was a short but succinct summary of the issues with using different subs, here's a slightly longer version.
- nearly all modern home theater subs are equalized
- they are equalized because the enclosures are small
- if they were not equalized then the result would be excessive bass roll off
- so what manufacturers do is get a long throw woofer, a powerful amp and some equalization
- they use equalization to boost the drooping low end, below the frequency at which roll off occurs
- equalization causes phase shift
- different subs have different amounts of equalization because they are using different drivers with different resonant frequencies, different enclosures (vented / sealed) and different sizes enclosures
- sealed boxes will have a different equalization than vented boxes as they exhibit different roll off characteristics (slope)
- therefore each sub model has a different amount of phase shift
- this phase shift starts where the manufacturer starts equalization
- some start at 50Hz, some start at 35Hz or elsewhere
- worst case you have full cancellation if the subs are 180 degrees out of phase at a certain frequency
- most times you will get constructive and destructive interference as phase shift differences are not 180 degrees but something a little lower
- there is no way to fix this with either the phase control on the back of the sub or the time delay on the prepro
It is basically unpredictable when you start using multiple different subs. Sometimes it may work fine, other times it will not. Simplest is to use identical subs, which will always work.