Any follow-up to your issues with pops in the speakers? I also have a Marantz NR 1501, and about a month or two ago, I started getting speaker pops. I have an Apple TV (and previously a MacMini) that inputted to my receiver via HDMI. I bought the receiver in approx. 2010 (maybe Fall of 2009), so it's about 3 years old.
I assume it's the receiver causing the pops, because the sound of the pop varies as I turn the volume up and down. I noticed that the pops decreased in frequency when I unplugged my pre-amp that I was using for my record player.
I'm kind of new to AV systems, so if there's some more extensive trouble-shooting I can do, I'm open to suggestions.