What is Floyd Toole saying about extra amplifier power and headroom?

The combination sounds excellent to my ears at low to moderate listening levels, but I notice a slight compression in the soundstage at higher levels.

That's what the OP originally wrote.  I understand that to mean the soundstage dimensions shrink at high volumes.  Is that an accurate understanding?  If you have a smart cell phone, download an SPL meter app and take a measurement of the volume level you start to hear this "slight compression in the soundstage".

As mentioned above, the compression you hear is more likely to come from the 7 drivers rather than the amp. The obvious way to find out is by trying a higher spec’d amp on your speakers -- if the compression is gone, go for a more powerful amp.
That should be your first stop.If it’s the drivers, you need to hi-pass the signal going to the magicos, either at ¬60 -¬80H (half or a whole octave above the speakers’ -6dB point). In other words, limit the amount of bass the magicos have to produce thereby reducing the strain on the woofers, and channel those frequencies to your sub.
Any active Xover can do the job, including the one you mentioned.


But it’s strange the drivers overload so fast -- you must be listening very loud :)

diamonddupree , before you spend $3000 on an active crossover to experiment with sub integration, buy a $149 Schiit Loki Mini tone control. It’s a relatively cheap way to experiment with reducing the driver load on your Magico Mini II speakers. You may find that you just need to roll off the lowest frequencies using the Loki and raise the roll off frequency on your JL Audio sub. You did mention that the Magico Mini’s have great bass extension (for stand-mounts) but if it’s at the expense of the rest of the music the point is somewhat moot.

The attenuation of the bass frequenciy range provided by the Loki may not be an ideal match for your speaker/sub pairing in your room but it will at least give you an idea of how the two interact. You might find that having the Fathom 110 provide more of the range of bass frequencies is a good thing. And may help in deciding on whether or not to purchase an expensive crossover unit like the CR-1..
For reference, I have stand-mount speakers similar to mapman and use two 10" sealed powered subwoofers in a 12’x20’ room. The subs are located at two different positions relative to the speakers. I have the Loki Mini tone control in-line between my preamp and a class D amp using one set of pre-outs and the subs are connected to the second set of pre-outs. I ended up rolling off the bass frequencies to the speakers by about 7dB and actually raising the roll off frequency on the subs to 90-100 Hz. Here’s a graph of the Loki’s tone control frequecies: https://i.imgur.com/XGDm140.png.
Just a suggestion for an inexpensive way to experiment with your setup, not necessarily a final solution.
Tom

@tketcham that looks great. This is definitely the way. I was also just looking at a MiniDSP unit. Is that the same as the Loki? It looks like an active crossover that controls roll off frequencies for anything connected to it. It's also super inexpensive. Thanks! 

@gregm I'm going to look at hi passing the Minis with an active crossover, perhaps the Loki that @tketcham is recommending or a Mini DSP. Both look like inexpensive ways to figure out what's happening. And to answer your question, yes, this is only at very high listening levels. My neighbors love me. 

@onhwy61 I'll try an app to test SPL. Wasn't aware of one so thanks!