I tried the OPA 1656 chip in the notch filter and it works fine.
Here's some photos of the attenuator I built based on 'O' or 'Box' pads. I recommend this type of pad as it is 'ground free' and so does not have any grounding funnies. My fancy 'scope which also does Bode plots, shows that it is flat from d.c. up to 5MHz (-0.1dB and a fraction of a degree phase shift) and can't measure the insertion loss as it is so low. The case is lined with self adhesive copper plate except for the front panel which is done with 3M nickel screening sheet, and the interlinks are done with 0.4mm silver plated copper wire. Not a sign of any mains or RF pickup.
All pin 1s on the XLRs are star linked and floating with an option to connect them to the case screen on the back panel. The front panel legend is rudimentary but works for me. The leads coming off the back of the PCBs connect to 'finger' tracks between each pad to add to the screening.
A couple of questions for you. What is the "dBc" measurement that you use - that's a new one to me - what is the reference level?
Secondly, have you thought of doing a variable frequency version. A 15k/30k 1% wire wound pot with a gearing mechanism would give a good range variation in the first instance. Coupled with an output attenuator it might be a commercial opportunity there for you

Cheers
Mike