Dual Velodyne DD+ subwoofers

[Delija]

The part that is incorrect is the bit about 99.9% of the subwoofer range being in mono. That's probably the case if you limit the upper frequency of the subwoofer range to around 80 Hz but many people run their subs to higher frequencies than that because they're using speakers which start rolling off at frequencies of 80 Hz or higher. In practice it's the user who limits the subwoofer range when they set the crossover frequency so it's the user who determines the subwoofer range, not the subwoofer manufacturer or the recording engineer who mixes the recording.

I would say it's far from "many people". If we are talking about Hi-Fi sistems, and we are, it's minority, but @Atomjohan is really one of them , since Raidho X-1 has pretty limited FR on lower side. I expected it goes lower than 80 Hz, but I was wrong - I just looked at the specs.

In his case, yes, he should use stereo subwoofer output on Devialet, since crossover must be higher than 80 Hz to get flat response in crossover area.
Of course, this makes sense only if his subwoofers are positioned close to main speakers - in stereo configuration and not in e.g. diagonal corners of the room.

...but with some other instruments like the piano which goes about half an octave lower than the bass the instrument including the bass notes seem to be mixed to an off centre location so their bass isn't present in mono. That's another reason I'm not certain that 99.9% of low bass is mixed to mono, a lot certainly is but definitely not all of it in my experience.

Nope. The perception of "stereo" bass comes from upper harmonics. It's really very easy to test this - turn off one of your subwoofers, and the bass will still be "stereo".

As I said, it is occasionally possible to determine the location of a sub because it is loading a room mode strongly, something corner placement does though other locations can also strongly load a room mode. If a mode is strongly loaded at a particular frequency then the sub can reveal itself as a separate sound source and you can locate it even though the mode being loaded occurs below 80 Hz. Also as I said, in my experience of a few occasions on which this has occurred, it has happened with loud low frequency sound effects in a soundtrack rather than with music so it may not be relevant in the OP's case but under the right combination of frequency content in the recording and a strong activation of a modal frequency by the sub, the sub location can be identified. This has not been common in my experience but it has definitely occurred on a small number of occasions and it isn't related to the directionality of low frequency sound, it's simply because some low frequency sound effects in movies are essentially the bulk of the sound when they occur so the sub or subs are the only speakers really working when some sounds occur and that can make it easier to locate the sub just from the volume level of the sound since there are no frequencies in the sound being reproduced by other speakers which are high enough in frequency to provide the normal directional cues. This is probably not a problem if you're only using your system for playing music but if it's also doing double duty for HT work the sub may occasionally identify itself as a source of the sound.

This should not be the case even with mono configuration with two subs (if they are not on the same place - stacked, of course). The fact that LFE in movies is always mono is not even important for this.

One case that crosses my mind would be low quality subwoofer with bad housing which vibrates and produces higher frequency "parasite" sounds.