Pop Sound in my speakers when driven loud from studio recorded CD sources.

If they are ported, seal the port with a rag. Repeat.

Yes, they're ported.  What will blocking the ports do??

Almarg, if you are out there I would like to know your full response. Not sure why it didn’t get posted here.

I deleted my post a couple of minutes after I submitted it, because when I composed it I hadn’t noticed your statements to the effect that the problem only occurs when the volume is turned up to loud levels. That fact makes what I had said in my post inapplicable, since it indicates that the cause of the problem is "downstream" of the volume control. FWIW, though, what I had said is as follows:

Given the 180 mv sensitivity of the Marantz receiver and the 2.1 volt maximum output level of the DacMagic+ unbalanced outputs, it seems very conceivable to me that the problem may be due to overdriving whatever circuitry in the Marantz precedes (i.e., is "upstream" of) its volume control.

As an experiment, and conceivably as a permanent solution, try enabling the DacMagic’s digital preamp function, reduce its volume control setting significantly, and increase the volume control setting of the Marantz to provide preferred volume levels. See if the problem still occurs.

Alternatively, you could try enabling the DacMagic’s digital preamp function and connecting it to the input jacks of the power amp section of the Marantz (initially with the DacMagic’s volume control set low!), while disconnecting the Marantz’ pre-outs from those jacks.

If one or both of those experiments confirms this diagnosis, an alternative solution would be to insert a pair of Rothwell attenuators between the DacMagic and the Marantz.

Again, none of this appears to be applicable, given the sensitivity of the problem to the setting of the volume control. Not sure what to suggest at this point, beyond what the others have said.

Good luck. Regards,
-- Al

Al,  Thanks for reviewing your original response.  Most here seem to think that the amp is clipping.  Is that what you are describing by saying: 

"...Given the 180 mv sensitivity of the Marantz receiver and the 2.1 volt maximum output level of the DacMagic+ unbalanced outputs, it seems very conceivable to me that the problem may be due to overdriving whatever circuitry in the Marantz precedes (i.e., is "upstream" of) its volume control."

On the other hand, Rodman thinks:

"...You might be hearing the woofer voicecoils, bottoming out, or- when woofers exceed their max excursion, things can sound really nasty...."

If the woofers were really bottoming out, wouldn't that mean the speakers were being seriously overdriven (too much power)?  Simao, disagrees with that and says this:

"...The Hales are 86db at 8ohms - that might be a pretty difficult load to drive at high volume for most amplifiers less than 100 watts..."

The Marantz was bench tested recently at about 85 wpc.  Therefore it may not be driving the speakers efficiently enough.

Am I understanding all of these comments correctly?  Thanks!

Al, Thanks for reviewing your original response. Most here seem to think that the amp is clipping. Is that what you are describing by saying....

No. What I was referring to in the post I deleted was clipping of the "small signal" circuitry in the receiver that precedes the volume control in the signal path, that is in the receiver’s preamp section. The others are referring to clipping of the power amp section of the receiver, that would result if the amp is asked to provide more power to the speakers than it is capable of providing. As I indicated in my previous post I deleted my earlier post when I noticed your statements that the problem only occurs at high volumes. If my hypothesis had been applicable the problem would manifest itself at any perceivable volume.

In any event, after looking further at the specs on the 2275 and doing some calculations, it’s hard for me to say whether the problem is inadequate amplifier power, resulting in clipping, or a defect of some sort in the receiver or the speakers.

On the one hand my understanding is that as a rough approximation rotary volume controls tend to provide attenuation at the 12 o’clock position (which is where you stated the problem tends to begin to appear, for those recordings which cause it to appear) that is roughly in the area of 20 db or so. 20 db of attenuation would reduce the 2.1 volt maximum output of the DAC to slightly more than the 180 mv that would cause the amp to reach its maximum power capability. Which in turn points in the direction of not having enough amplifier power.

On the other hand, according to the specs I’ve looked at the amp is rated to provide 75 watts into the speaker’s 8 ohm nominal impedance, and you indicated it was recently bench tested as providing 85 watts. In a medium sized room at typical listening distances (say 10 or 12 feet) 75 or 85 watts into two 86 db 8 ohm speakers should be able to produce sound pressure levels of around 100 db or so at the listening position, assuming the speaker specs are accurate (although that is certainly not always a good assumption, particularly when it comes to speaker specs). SPLs of 100 db or so are more than most listeners would require on most or all of their recordings, the most likely exceptions being recordings having exceptionally wide dynamic range (i.e., very large differences in volume between the loudest notes and the softest notes, such as some well engineered minimally compressed classical symphonic recordings). But in most cases the ability to produce that much volume at the listening position would suggest that amplifier power is adequate.

So I’m not sure what to point to as the likely cause of the issue, aside from the probability that it is being introduced downstream of ("after") the volume control in the signal path.

Regards,
-- Al