I think @Innocent_Bystander is far better at this than me.
I have tried Spectroid on three devices and they do show different results, I think phone mics, if they are optimised at all would be for voice, so above 100 Hz.
You are right. The performance is limited to the phone mics. But Iâve tried it with an external AKG K414 mic, which has a response frequency of 20hz-20khz. The curve looks the same. If you compare the first plot without the subs, with the rest with the subs, you can see the lower 30hz to 50hz was definitely boosted and picked up by the phone with the subs. So I think nowadays the phone may be a quick and good enough tool.
Iâm quite amazed of the phone mic results as well when I hook up with an external mic to test and found the results are quite similar⌠I guess when they improved the cameras on the phone each year, they didnât forget to improve the mic as well. 
The phone/app could be good enough, but it would be far from surprising if it doesnât give the best results, however for your purposes it might do.
Is that AKG microphone a measuring mic? Few mics have a flat response across the audio spectrum, even when they claim to go from 20 Hz to 20 kHz and even might claim a response within, say, + or -3 dB. Mics for vocals almost invariably donât have a flat response.
I am currently using Confidence C2 Platinum, which reach deeper than the Contours to 28 Hz -3db. I have each sub set at the lowest crossver (i.e. 0 clicks) and the gain at 9 clicks.
Volume level on my 252 is same as it was without using a sub, right at about 9:00 for moderate listening volume. The subs are connected directly to my speaker terminals. On each one the red/yellow leads are combined and connected to the + speaker termimnal, and the black lead hangs free (unconnected).
When I added a second sub I might have reduced gain by a click oir two, but certainly not by half. When I had only one sub both speakers were connected to it, so it got singal from both. With two subs each gets signal from only one speaker.
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interestingâŚany reason you left them unconnected, instead of connecting to the - terminal?
Why would you leave black unconnected. The red and yellow are the L and R signal and black is the Earth common.
So if itâs not connected you donât have a circuit. It should be connected to a -ve speaker terminal
My dealer says thatâs correct, that for two subs connected high-level to my speakers to leave the black unconnected, unless you get hum, in which case you should connect them. I also found it printed on RELâs site somewhere, but now I canât find it in their connection guides. The Red and Yellow are summed together and connected to speaker plus because I have one sub for each channel. There is definitely a circuit because I have output from the subs.
Itâs effectively the same as RELâs guide for connecting a pair of subs to a pair of mono bloc differential amps, and my dealer says that is the same issue regarding hum. If you get it then connect the black leads to speaker -.
I have done both and cannot tell a difference.
It sounds like a series connection of the channels, but it makes the assumption that the ânegativesâof each speaker are connected to one another, which with most amps is true as the ânegativeâ outputs are both grounded. But not so for balanced amp designs, like the NAP500, when I donât think youâd get any output that way - and you could well damage the amp if you connected both speaker ânegativesâ together.
I have a 300DR and it works as I described. I imagine the new pair of S/510 that are on the way will connect the same way.
I might use one of the old R-328 in my office system with a Atom and pair of wall-mounted Dynaudio Audience 52SE.
I suspect itâs making a circuit via mains earth.
Sounds a bad idea to me, but it evidently works well enoughâŚ
Reflecting on it, it wouldnât be a series cannection as that way it would be out of phase and lose any signal common to the two channels. So indeed it must be getting the ground connection for each channel via mains earth connections - again wouldnât work with bridged amps (e.g NAP500) as neither side of the outputs is connected to earth.
With a bridged amp, the sub would likely see half amplitude if referenced to mains ground.
If I connect the black Neutrik leads to speaker - terminal I donât hear a bit of difference to when I leave them hanging free.
Were you expecting to?
No, but I just thought Iâd double check, based on discussion above. My dealer says one might need to connct them to resolve hum, but if that isnât an issue leave them disconnected.
I think itâs likely to be dependent on the grounding scheme of the connected system. Naimâs scheme âexpectsâ the signal ground to be connected to mains earth in one place only - hence all the fun with âdefaultâ and âfloatingâ with their source components - but it is connected (Iâd guess in your case through the Superline); I suspect the RELs would not work at all with the black hanging loose if the driving system was double insulated (ie two pin plug or earth pin not connected).
Well, I have a Superline and a NDX2, and they arenât both always on, so I guess either the Superline or NDX2 or both provide that ground ?
Unless you unplug them, the ground connection is still there. The NDX2 has a switch on the back which should be set to âfloatingâ if Iâm correct about the Superline, otherwise you have two links between signal ground and mains earth.
Given the âofficialâ line is (I think) âset the switch to whichever position doesnât hum and/or sounds betterâ it probably doesnât matterâŚ
(Naim CD players always had a permanent link to chassis, which Iâm extrapolating to the Superline as another source component. I donât know.)
My NDX2 is set to âfloating.â I donât remember if it made any difference when I installed it, and I have since added a XPS-DR, and donât recall checking or changing it at that point either.