Woofer Flutter

             Assuming you're problem is via Phono only and (hopefully: 47k Ohm) MM Input:

         https://store.acousticsounds.com/d/60042/DB_Systems-DB_Systems_Subsonic_Filter-Turntable_Accessories

             Can't say how that might affect your presentation.

             Most phono stages that filter subsonics, do it the same way, but: internally.

KAB Rumble Filter, $179.95.  Best investment I have made. No more woofer fluttering.  Designed with all audiophile grade parts. 

        Didn't see a processor loop (or any other option), available in the 7900, in which the KAB could be inserted.   

                                But: I could be wrong.