Calibrating stereo subs

Background ;
Having just bypassed my preamp and gone straight into mono amps from my DAC.
The problem is that I need pre out to my active subs in addition to line out to the fronts.
I do not want a split-cable solution, so what I do is to use both RCA and XLR out from the DAC.
So far so good, but the fact that single ended output (RCA towards sub) uses one side of the balanced transformer, we get 6dB lower output than the XLR fronts. The load the RCA output observe is 47kohm, so it is high enough to not pull down the signal on the outputs. Rout of DAC is 1,5kOhm.

My idea was to adjust the volume locally on subs to compensate for -6dB level difference, which should work .
There is no scale for the volume, moreover, the gain adjustment scale is not directly transferable to experienced SPL .
Fronts has 2 pieces. 12" woofers and sub ​​has 1ea. 12" .

On this basis, I isolated the front left and entered a 45Hz sine (XO at 50Hz , 24dB / octave ) with 80dBc SPL in the sweet spot . Then I muted fronts and adjusted sub to the same SPL as the fronts .
Did the same for the companion channel.
Result: Too hot reproduction <50Hz.

In the next test I adjusted the subs so they matched fronts, i.e. I measured the fronts and stopped raising the volume bar for the subs when I saw a level elevation of 1/2dB.
Result: Missing depth, impact and substance as fronts dominate <50Hz.

Finally, here comes the question for counseling;
Is it possible to calculate how to adjust this based on measurements?
When the speaker membrane surface area increases (moves air) , so does the SPL.
What I am looking for is a approximate number that I should adjust the subs, based on the given SPL from the fronts.
For example, if the fronts provides 80dB at 45Hz, what level should we expect when the subs engage (e.g. '83’dB)?
An auto calibration (mic setup) won’t work this late into the loop because subs and fronts play simultaneously, but there is not only one of the signals that need to be adjusted.

With my Avantgarde Duo Omega’s I use the Dayton Omni Mic measurement kit to make adjustments.

The Duo’s are a difficult speaker to adjust by ear.

I also have deleted my pre-amp from the equation.

My pre had dual XLR outputs, one for the main amps and one for the SUB225s.

I went back to sending the speaker level signal to the subs instead of using the XLR input.

My room is an asymmetrical nightmare.

The crossovers, sub level settings and balance all had to be set differently for each channel and the Omni Mic was the only way to do it accurately.


I guess putting my room properties into a DIY/DSP software and simulate the response won’t do the trick.

This, actually was the solution I had hoped for.

Yep, reviving an old thread:

I know many advocate stereo subs. I would encourage those with 2 or more subs to run them in mono, pick a channel, doesn’t matter and send them all the same signal.

Why? One can now use constructive and destructive phase adjustments to smooth the response throughout the room. When feeding a stereo signal, the wave can vary enough to lose much of what is gained by having them play the exact same content.

My subs are arraigned one on the front wall midway and one on the back wall midway so stereo really has no value in my space.

Running mono was an obvious improvement for me. Bass is tighter, more controlled, more articulate and smoother.

Thank you for this, Brett - really interesting and worth a try!

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Another that supports mono signal

I spent less than an hour setting gain, crossover and phase to match my mains today

Using RTA and two subs (or more) one can rather simply dial out much of the effect of room modes

I’m sure I’ll work to dial it in even more in the future but it sounds so good now I just have to listen