I have a Squeezebox that I use with an external DAC (Lavry DA-10) and find that a wonderful combination.
The DAC that is built-in to the Squeezebox is good (especially for the money) but easily improved on. The Squeezebox is very receptive to an external DAC. I don't know anything about your Theta Pro but if you like it now, you should like it with a Squeezebox. If, perchance, you happen to prefer the SB's internal DAC, then all the better. You have a DAC you can sell to recoup some money.
I don't have an opinion on the speakers you're suggesting as I'm not familiar enough with either them or your preferences.
The DAC that is built-in to the Squeezebox is good (especially for the money) but easily improved on. The Squeezebox is very receptive to an external DAC. I don't know anything about your Theta Pro but if you like it now, you should like it with a Squeezebox. If, perchance, you happen to prefer the SB's internal DAC, then all the better. You have a DAC you can sell to recoup some money.
I don't have an opinion on the speakers you're suggesting as I'm not familiar enough with either them or your preferences.