I stand corrected - I've not used a Garmin x96 that much, so obviously missed the feature.
My Aera 500 uses an internal antenna - not sure if there's an external option, and it's pretty good, but yes - not flawless - I have certainly seen dropouts where I didn't with a panel mount + external antenna on the same aeroplane.
Just reading through the full spec on the 660, I think I was starting to drool and had to stop reading !
G