Center channel speaker intermittently quiet

Receiver: Marantz SR 6011
Center channel: B&W HTM72s2 8ohm, min 4.3ohm and sensitivity is 87dB
It's a multichannel setup- I have some small satelite speakers (Bose Acoustimass cube speakers) x4 + 4 installed down-firing in-wall speakers similar to these


The problem happens on all sources- TV over ARC, AppleTv, FireTV, cable box, blu ray player. Basically, the center channel becomes very quiet. It does not cut out completely. The surrounds seem to continue to play. It most often happens while streaming TV shows (not during intense scenes where all 9 channels are noticeably stressed/firing. It is most noticeable when there is audio and typical background noise from an outdoor scene. I don't think it's the actual track based on how it happens- sometimes the same character is speaking while the audio drops then returns. But the center will become very quiet for a few seconds, up to about a minute, maybe two and then suddenly be back at full blast. When the volume is turned up, everything rises in unison (it doesn't "snap" the center back to loud. It becomes louder, but only at the appropriate rate vs the surrounds)


What it is not:
I have no correction/auto-sensing or damping features on the receiver turned on
any specific source
any specific scene
It isn't any particular time after it has been on
It sometimes happens every 10-15 minutes. Sometimes it doesn't happen over the. course of a full movie.

If I were betting, I would think it is the amp in the receiver. But I'm no expert, and without spending money, I have no real way of testing so would like to avoid that unless necessary. It isn't unbearable, but it is starting to be noticed by the wife, so I would like to take a step or two in the direction of remedy.
1) Any thoughts as to why something other than the power amp would occasionally do this (it IS in a cabinet, but the cabinet is fairly large and has decent air movement. It is not particularly warm/hot
2) Should I just go ahead and buy a Dayton APA150? The receiver has an out for each channel, so dropping it in-line would be easy


This isn't my "big rig" but does get used in my house more often than anything else. If it sounds like a receiver replacement (because this is further up the chain in the receiver) or something that a faulty speaker does/would do (and that just doesn't make sense to me, though I am but a novice), I wouldn't have a problem just replacing the whole thing, but if this sounds like a power amp problem (which is what I am telling myself it sounds like, but I don't have a real wonderful basis for this and have not found anyone complaining of a similar thing online).

I think I'd rather go with something like this because I could/would have other uses for it if it doesn't fix the problem, but I also don't need it if someone with a little know-how doesn't think that could be to blame at all.