I went and looked at an old job that has an AVR-2807 with an XM Mini tuner & dock, which is hooked to an outdoor antenna. The customer was saying it constantly said CHECK ANTENNA and has for months.

I unplugged the outdoor antenna and plugged in the standard little antenna that comes with the unit. No change - but after I power cycled the AVR and moved this antenna to the window, music started to play. It was choppy, but it worked. I then unplugged the little antenna and reconnected the outdoor antenna. The music never stopped (i'm assuming it had buffered in its memory) but the reception improved and it kept playing...

...for a few minutes. Then CHECK ANTENNA came back. So, I repeated the steps but left the little antenna hooked up for longer. Eventually CHECK ANTENNA came back with the small antenna hooked up.

So.. I'm not exactly sure what the CHECK ANTENNA message actually means other than "there's a problem!" I'm thinking the culprit is likely the XM mini dock itself and neither of the antennas, but I'm not confident in that.

Anyone have experience troubleshooting these? I haven't touched one in years.

Also, XM sound quality is awful.

My experience is that you are correct that the mini dock thingy is probably flaking out.

I have a little portable XM unit that I use for this. Plug into antenna, see if you get reception.
It doesn't have to be activated to verify the antenna.
Have seen a few of those outboard tuner modules go bad. Heck, I have seen a few of them bad out of the box.

And yeah, XM sound quality is only slightly better then Tune In Radio. For sure a house audio only source, not for the discerning audiophile.

I have one of the XM Mini Docks that I don't need if you decide to replace it - cheap.

You can go into the xm menu to antenna pointing. See if you have ample signal before dismissing the unit. Then add Sonos if needed.