REL six pack questions

I can’t speak to your technical questions, but I can give you my low-tech experience with my own Rel stacks (3 S 501’s on each side).  I set up the stacks and then tuned each by ear, starting with the floor and moving up to the top, one at a time, turning off the other 5, but adjusting the pairs together before moving to the next level.  I run them off the High Level outputs.

As to your other questions, I can tell you what I hear, but do not have the expertise in acoustics to explain why or how it works.  First the bass is very tight, controlled, and fast, with no boomy, bloated low frequency thumping at all.  There is a better sense of space around instruments and vocals, and a far better sense of weight and presence (but not unnaturally  up front) with each note.  The sense of space is three dimensional, which I think of as the polar opposite of, for example, the Phil Spector Wall of Sound effect.  The sound stage is improved in all aspects.  I do not notice any nodes or suck-outs, but I imagine that would vary across listening rooms.  The overall effect brings the sound much closer to an actual live music than any other component upgrade I have done.

*S510’s

Thanks Charlie. I keep reading these kinds of things, so am excited to hear it.

Your observations are very astute.

1. I agree with you, I would have all units in place after the bottom one is properly located, and make the initial adjustments individually.
2. 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.
3. 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.
4. 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.

Thanks audiorusty! Yeah you make a great point about them using small monitors for their demo. But in many installations they use giant main speakers that have response down to the bottom. So it's still a mystery to me with those kinds of speakers, or even with mine that go to 28 Hertz, how adding more response in let's say the 30 to 60 Hertz range doesn't just turn into a frequency imbalance. Unless maybe it's related to our ears natural insensitivity to low frequencies... Anechoically a speaker could be flat in the low frequency areas, but perceptually they probably have a lot of roll-off to our ears. Maybe that allows these subs to add energy without creating what seems to be a great imbalance. No idea if this is right or not but I have not heard anyone explain it In a way that makes sense to me! 

Thanks again for the thoughtful response!