Subsonics with Grado & Rega Arm

I am currently using a Grado Gold with a Expressimo modded
RB-300 on a Thorens 320 suspended turntable with very good
results.I'm suspicious that there is something else that
has not been investigated.Have you verified that all of your
wiring(low volt. vs. 120V) is truly isolated ?

Not sure I understand what you mean

I was having a similar problem. I have been using a Grado Reference Sonata with my Rega RB300 Origin Modified arm on a Sota Star Saphire table with the Sota Reflex clamp. I sent my arm to an Audiogoner (Michael Wharton - teatime66@hotmail.com) for rewiring the arm in Cardas and installing a Cardas 5 pin DIN termination. I purchased a custom tonearm cable from a terrific fellow in China that incorporates twisted pair construction of LC-OFC wiring with braided copper shielding, hermetically sealed insulation and an RF stopper - all for a very fair price and QUICK turnaround). The problem is completely resolved. I am hearing much deeper into the vinyl now even before any meaningful time for burn-in has occurred. I could not be more pleased. Actually, this is one time I can be the guy to say that he cannot believe the magnitude of improvement a tweek made.

Would a phono stage with subsonic filter help?
Grado dance and woofer pump when used on an old Thornes. I dont think Grados like suspension turntables. No dance on rega table but hum increases nearer to motor. I think Grado on Rega is very musical regardless of how the numbers crunch out.

A tonearm rewire would probably remedy this situation. A typical 3 wire set up is required for a Grado. Without a dedicated ground wire problems like those posted happen.