Scoly1, I read your original posts a few times just to make sure I understand what you are asking for. I've not used a squeezebox myself, but based on the pictures I found online of the unit, I think this is how it works.
Currently, you are probably connecting squeezebox to your DAC via the digital output (orange) connector on the squeezbox. It looks like the squeezbox also has an internal DAC, so an alternative is that you can connect the squeezbox directly to a preamp via a pair of RCA cables. The reason that the preamp does not have a digital input is because most preamps only accept analog signal. So if you have a digital source, you would need a DAC to convert the digital signal to output to analog signal. Most digital source has either DAC built-in or would need to connect to an outboard DAC via a digital output/digital cable.
What are you connecting the DAC to currently? Is it connected to an integrated amp (is there a volume control on your amplier) then to the speakers? What's the reason you are adding a preamp?
Remember at one time all electronics are based on tubes. Many audio gear today still are. So, having tubes in both your DAC and preamp, and even in your amp, would not be an issue. As tubes are easy to change and will alter the sound characteristics, you now have the ability to change tubes in your DAC as well as your preamp to your liking.