Are you sure the preamp is turned on?

Here’s more on what a preamp does: https://en.wikipedia.org/wiki/Preamplifier

You definitely want to turn it up from the default. I have mine set at 8. But 8 to 10 is good. 

Thanks for the info. The preamp is built in and automatically on when the turntable is powered up - there is no instruction issued by the manufacturer for doing any additional steps so I don’t think that’s the issue. 

You probably need to adjust the input volume setting on the SPeaker the Line in being used..  For Turntables it’s normally 9 or 10, default is 2..  CAn’t see the screen

 

Thanks. I will have a dig around in the settings for this. Can you send the bread crumb trail (ie “Sonos app - settings - sources - “) so I can follow it through please?

Looking at this webpage, there is no indication of a pre-amp inside this device. I’ve never heard of one without a switch, to turn it on or off, either. 

Rega P1 plus, as quoted in OP:

Planar 1 Plus

All the features of the award-winning Planar 1 turntable with the addition of a built-in high quality MM phono stage, making this the ultimate ‘Plug and Play’ turntable.

You probably need to adjust the input volume setting on the SPeaker the Line in being used..  For Turntables it’s normally 9 or 10, default is 2..  CAn’t see the screen

 

Thanks. I will have a dig around in the settings for this. Can you send the bread crumb trail (ie “Sonos app - settings - sources - “) so I can follow it through please?

Found the setting.  Thanks. 

The manual states that there is a pre-amp phono stage built in. It talks about the delayed start up of the phono stage as well as warning to not plug the leads in to a phono stage on an amplifier. All indications are that the pre-amp is not switched therefore one cannot use a 3rd party pre-amp.

https://www.rega.co.uk/download/D1007MAN-A00-4-website.pdf

There is no indication in that text of a pre-amp. Merely a phono stage, which comes ‘before’ a pre-amp.

Wording confusion here, a Phono Stage is equilivant to a Phono Preamp depending on who's version of English you speak. 

A turntable with a Phono Stage would connect to a normal receiver's line-in and not the Phono inputs.

Does the CD input sound good? If so try swapping  the turntable and CD input connections.

Is the cartridge properly mounted, height, tracking angle, weight, anti-skating? 

Nope, a pre-amp is not needed for Sonos.

A phono stage or phono pre-amp is and the terms refer to the same equipment and so are interchangeable.

The TT, as stated above, includes a line level phono stage (or phono pre-amp if you prefer).

Let’s clear this up regarding phono stage vs pre-amp. The terms are being used interchangeably. Below is a page from the manufacturers manual. The first sentence (if you can read it 😂):

OK I give up... Look at the tech. specs. for the Project TT's that Sonos themselves sell through thier own website... which state:

* Built-in phono preamp/phono stage

I guess anything that amplifies a signal before a power amp is a pre-amplifier, although context dependent 🤓

Just like line/instrument/microphone pre-amps. 

The key is a Phono Stage / Preamp has an RIAA equalization curve circuit that removes the RIAA equalization that is applied before the record is cut.

Yes it (whatever you call it) changes the signal levels but the key function is the equalization, makes it useless for other tasks essential for playing a record.

Thanks all for the help. The Sonos setting changes have made a major difference so I think that was the problem. 

LP surface noise can be significant and it’s mostly high frequencies. But, there is relatively little energy in the highs.  Bass, on the other hand, has lots of energy. A problem with heavy bass notes is that the disc cutter may slice through the groove wall to the adjacent groove. This results in an impossible to play track.

During LP mastering highs are boosted and bass is reduced. This is RIAA Equalization. On playback inverse RIAA is used to restore the frequency response. As the highs are reduced, record surface noise is also reduced and bass is returned to its correct level.

Before the early and mid 1950’s record companies would each use their own equalization. This resulted in general purpose phono-preamps offering a choice of equalizations. This was a mess for the operator when playing a stack of records on a changer because equalization might need to be changed for each record. During that era we also had “ceramic” phono cartridges that did not need a phono-preamp.