Just connect the Theta's RCA outs to the Aux in on your amp, assuming your CD player still occupies the CD in's on your amp. Quite honestly apart from Phono in and Tape out all the other RCA in's on your amp or preamp are fair game.
You did not mention a pre or integrated amp so i assume you will be using the volume control on the Squeezebox
You did not mention a pre or integrated amp so i assume you will be using the volume control on the Squeezebox