Pairing Magicos with a sub

I found lots of discussion around subwoofers, but in most of rhe cases they are about pairing a sub with speakers with limited bass extension. I am curious does anyone have experience pairing Magico S5 or similar speakers, which go quite low, with a sub(s). I have the S5 and I am wondering if I add a sub can I improve the dynamic when listening at low levels. Although the S5 is pure joy to listen at higher volume levels, often I feel to tired for this and prefer enjoying not very loud music. This is the only reason I started to think adding a sub. After some research my potential choice is the Rel G1 Mark II. 
I will appreciate your thoughts on this.
You need a loudness control, not a sub.
Right. Us humans don't hear all frequencies equally. We hardly hear low frequencies at all until they get fairly loud. Then when they do get loud we hear changes in bass volume even more than midrange. This phenomenon is represented graphically by Fletcher Munson curves
What this means in practical terms is what Erik said you need a loudness control to turn up the bass so you can hear it at low levels.

Of course you are probably not gonna do that. What you might do instead is get a sub and adjust it with one level set high for when you listen at low volume, another marked for a lower level when you listen louder. Or just turn it off.
Thanks millercarbon, this is exactly what I intent to do - use it just when listen to low volume. Loudness is not an option with my pre amp / mono blocks and I am not keen of inserting an EQ in the path. Interestingly with the S5 at low volume I am loosing mainly the mid bass, not the low one. I guess the sub should do a good job to improve this.

I suggest reviewing the Magico Qsub setup process.

There are non Magico subwoofers available that provide a similar level of optimization and remote control at less cost than the price of the sub your already considering.

Equalizing a subwoofer will not affect your S5s.   
Magico makes the best commercial subs you can buy. Stick with them. Why corrupt great speakers with subs not up to the job. Will it increase dynamic expression at low volumes? You bet. Ideally you will need two of them but you can start with one especially if you are in a small space.
Back in med school I was in a high rise apartment building and obviously could not play loud.The addition of a sub woofer made music much more enjoyable at lower volumes. I could feel the music again.