I've attempted to replicate this issue your having as far as audible interference during playback/pausing and I notice a slight click as the audio is halted due to the pause function. Generally I have always noticed a small audible click when pausing with any A/V receiver I've ever owned, but I am unable to replicate the clicking when adjusting the volume on my 975 in either direction when paused, or when no music is playing. I have tried it on all of my inputs in this scenario without hearing what your unfortunately struggling with.
As to sound coming out of the speakers while headphones are plugged in, I noticed this also, but the sound coming from the speakers is minuscule by comparison to what is coming from my headphones. Still I agree that this isn't quite normal, but my regular listening habits don't normally include a lot of headphone usage so I'm not as concerned about this. Please don't despair, your concerns are quite valid and as Peter mentioned I'm sure they will be able to assist in resolution of you problem very quickly rbpett! Good luck my friend!
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Home Theater:
Vizio M70-C3 70" 4K TV
Outlaw 975
Outlaw 7125
Panasonic DMP-BD85K blue ray
Xbox One X 4K Blu Ray/Games
Sony SS-K90ED tower speakers
DefTech Mythos 3 center
Modified Pyle 3" 2-way cube speakers side surround
Insignia 3" 2-way bookshelf surround back
Modified Audio Source SW-15 subwoofer (downward firing) 3000 watt
Dayton Audio SUB-1200 12" Subwoofer