Your observations are very astute.
- I agree with you, I would have all units in place after the bottom one is properly located, and make the initial adjustments individually.
- All over lapping frequencies reproduced by different drivers would have to be in perfect phase alignment in order not to be a muddy mess, though it might be possible that due to the latency of the passive radiator in the REL subs, this issue might be masked to some extent. You will notice in the video that A) they are pairing their six-pack with a bookshelf speaker that probably doesn’t have any meaningful bass below 60 Hz. And B) they don’t play any music through the system at any point in the demonstration.
- There is no such thing as hidden bass in a recording, it’s whether or not a system can reproduce the bass that’s in the recording, not all systems can, which is why subs exist. If your mains can reproduce low bass with authority, there is very little reason to add subs, unless you are looking for more impact, in which case there is very little reason to have mains that can reproduce low bass, in my opinion.
- They do not. The harmonics are created by the instruments and are captured on tape or digitally in the recording and then reproduced by the driver that reproduces that particular frequency of the harmonic during play back. So even if your system is unable to reproduce the low bass fundamental of an instrument the harmonics of that instrument are still captured in the recording and your system is currently reproducing them during play back.