I’ve never used a Squeezebox Touch myself, but taking a look at the manual:
The back of Squeezebox Touch has both analog stereo (the red and white RCA jacks) and digital (coaxial or optical) jacks.
You can output your music from Squeezebox Touch to a stereo system, high-end AV (audio/video) Receiver, portable stereo system, or a pair of PC speakers. Any stereo or set of speakers that has an auxiliary input jack should work.
You can also plug headphones into the headphone jack in the back of your Squeezebox Touch.
Digital inputs 1x USB (B), 1x Co-axial (RCA), 1x Optical (TOSLink)
So it looks like you should be able to connect the Pre Box to the Squeezebox okay. The Squeezebox would be the endpoint in Roon and you’d just configure that to play to the Pre Box.
One thing I didn’t mention in my last post:
If you’re referring to the Pro-Ject Pre Box DS2 Digital, this is Roon Tested, not Roon Ready. Roon Ready devices can be streamed to over the network. The Stream Box S2 Ultra from Pro-Ject is a Roon Ready bridge, if that’s what you were looking for instead.
You can read more about the different certified Pro-Ject devices here: