I agree with your idea of using the Phono Out.
If the Phono Out of your preamp indeed represents the output of your preamp's phono stage, presumably the Phono Out puts out a line level signal (unimpeded by the volume control) of whatever you're playing on the turntable, all the time. This would be appropriate for the equalizer. Accordingly, I would connect the Phono Out to the input of your equalizer. I would then connect the output of your equalizer to an Aux input on your preamp. To play music using your turntable through your equalizer, I would specify the Aux input on your preamp's selector control. Of course, this won't help if you want to equalize another source signal.
I also agree that there could theoretically be some degradation of your signal from the conversion to digital then back to analog. Hopefully, this won't be really noticeable. The equalizer may have a defeat switch so you can A-B compare the sound with and without the conversion process. If the change in the sound quality bothers you, I would send it back and either do without equalization or try an all-analog equalizer. (There are quite a few to choose from.) Good luck.
If the Phono Out of your preamp indeed represents the output of your preamp's phono stage, presumably the Phono Out puts out a line level signal (unimpeded by the volume control) of whatever you're playing on the turntable, all the time. This would be appropriate for the equalizer. Accordingly, I would connect the Phono Out to the input of your equalizer. I would then connect the output of your equalizer to an Aux input on your preamp. To play music using your turntable through your equalizer, I would specify the Aux input on your preamp's selector control. Of course, this won't help if you want to equalize another source signal.
I also agree that there could theoretically be some degradation of your signal from the conversion to digital then back to analog. Hopefully, this won't be really noticeable. The equalizer may have a defeat switch so you can A-B compare the sound with and without the conversion process. If the change in the sound quality bothers you, I would send it back and either do without equalization or try an all-analog equalizer. (There are quite a few to choose from.) Good luck.