I’m a tad confused. Where does the 5CD changer fit in? 

I think you’re correct connecting the Turntable to the pre-amp, assuming the output of the turntable isn’t already at line level. The pre-amp would be connected to the line in on the Sonos CONNECT, and the the line in settings would be adjusted, per the information found in this FAQ. Many people have found variations in the line in volume setting to be significant, you probably want to try several.

If the turntable is already pre-amped, it would have a switch that likely says either PHONO or LINE. If it’s already set to LINE (that switch can even be hidden under the platter!), then you don’t need the pre-amp, as the output is already Line level, which the Sonos is expecting. Adding a pre-amp to that would just ‘blow outs the sound. 

I’m planning on connecting the CD player to the system through the Connect box as well.

Thanks for your reply!

I forgot to mention that my connections are all RCA jacks.

You’ll need some sort of A/B switch for connecting the two devices to the single set of RCA inputs on the Sonos CONNECT, then. 

Right. I’ve been doing that with the powered speaker right now. Thanks! I’m wondering the order of connection for the RCA cables in that situation. Turntable and CD player to the A/B switch, A/B switch to the preamp, preamp to the Connect box?

No, the CD player is already at line level. The turntable to the pre-amp, the pre-amp and the CD player to the switch, the switch to the Sonos.

You might want to take the easy route first, skip the turntable and hook the CD to the Connect (no preamp issues) and get it working well.

Then try the turntable. You want to go from the cartridge to a PHONO preamp (a normal / non--phono one won't work) then to the Connect. Must have one and only one phono preamp in the circuit, internal to the turntable or external, either will work. 

Bruce and Stanley_4, thank you for your expertise. I look forward to putting your suggestions into action.