Distorted sound-Sonos Port

Additionally, when I stream music from Pandora, SiriusXM, or SoundCloud, there's no distortion. It's still being powered by the Acurus (obviously) which makes me think the issue is with the Line In feature.

Okay, so the Port provides volume control. Fair enough.

If other sources are fine through the power amp then the Line-In does indeed sound like it’s the issue. Source Level 1 is the lowest sensitivity, so ought to be fine with any standard line-level signal. I couldn’t readily find a detailed spec for that ancient CD player. You are using the unbalanced RCA outputs I assume?

You could independently test the Port’s Line-In, e.g. using the headphone output from a phone. 

Have you tried simply inserting some attenuation in between the CDP and the Port? Either a passive volume control or in-line fixed attenuators?  

It's definitely ancient but I've tried other CD players and nothing sounds as good!  Yes, using unbalanced RCA outputs.  I have the tech specs if there's something that would be helpful. I'm not smart enough to know how it would impact the Port so many thanks again for your help!

I have another ancient piece of equipment which is a passive volume control that can act as a pre-amp. When I hook up the Amplifier and CD player to it, the sound is incredible. BUT, even on  "1" it's really loud and obviously no Sonos capability.

Any chance you think it's just a bad unit with a problem in the Line In circuitry?

I was curious as to the nominal output level of the CD player’s RCA outputs, to see whether that could explain the behaviour. 

Have you tried inserting your ‘ancient’ passive volume control between the CD player and the Port? 

First time is a good reason/excuse. You will not get better support than what you are getting on this issue from anyone that I know of in Sonos that posts here. It probably won’t even be as good. 

One reason why you see some high post counts among some users here is because Sonos is often not to be seen - or seen in time - on many threads here.

Or such users were moderators in an earlier incarnation of this community. 

Jumping in because I did not see an answer to the quoted. This is an easy way to test the line in circuitry of the Port. Have you done that?

The tech spec for the RCA out of the CDP has two numbers - 3 volts and 6 volts. I don’t know if this means that the minimum is 3; if so that explains to an extent why it goes so loud, but from the little time I spent looking at reviews I did not see this pointed out as an issue by users as one would expect. And I would expect setting Sonos line level to 1 should fix the problem. Unless something is wrong with the Line In, which a test with any phone should throw up.

It looks like 3V is for unbalanced, 6V for balanced. At 3V it could be in danger of clipping on the Port’s ADC, since the Level 1 Line-In spec is 2.2V.

I invited the OP to insert his existing passive volume control into the line, to attenuate the signal and see if that helped.

Really appreciate the perspective!