Fixing the suspension of the Pro-ject RM6-SB elevated the sound of the turntable to a different level (in my system). I upgraded to the Pro-ject from a Thorens, but after a couple of weeks began to notice the rumble. I started googling for answers with hum, noise, rumble searches. Found a bunch of postings of customers complaining and also looking for answers. I went the route many Pro-ject owners suggested by isolating the TT. I mounted the TT on a shelf attached to a wall. Then I placed a mat between the subplatter and platter, and between the cartridge and headshell, and between the platter and the record. Then I purchased a set of threaded brass footers, then a silicone dot above the spindle (between the subplatter and platter). Each fix helped improve the sound of the turntable. The greatest improvement was the footers. Coupling each 5 pound "threaded" brass footer to the plinth made the biggest noticeable improvement. I recommend that upgrade to all Pro-ject owners. But even adding the additional 15 pounds of mass coupled to the plinth did not get rid of the rumble because I later discovered that the source of the rumble was from the vibration in the motor being transferred to the plinth. Thanks to the Applechap, I discovered the answer was simply and inexpensively to isolate the resonance caused by the motor vibration. What really convinced me were his before and after graphs; evidence that it worked.
I did not use the Henley fix, instead took Applechap's theory to further develop his original idea, which I felt was almost but not quite there at reducing vibration coming from the motor. I ordered a packet of grommets (http://www.grainger.com/Grainger/Rubber-Grommet-3MRT4
) which cost a total of $8 including shipping (I have about 40 grommets leftover). Purchased a round belt from Turntablesneedles.com for $19. The theory behind using the round belt instead of a flat one is because it is stiffer and there is less surface area touching the sub-platter to transfer resonance.
Installed the four grommets around the collar of the suspension screws. Then two more grommets, one above and one and below each of the two packing screws (I used a longer screw). The eight grommet method seems do the most to isolate the motor from having direct contact with the plinth (in my TT). I then attached the suspension o-ring belt between the lips around each of the four grommets (suspension belt must not touch the screws directly or the motor will continue to channel its vibration onto plinth).
The source of the rumble in these Pro-ject suspended motor turntables come from the four screws attached to the plinth having direct contact with the o-ring band. Some including the Henley aftermarket kit, have only used two grommets where the packing screws hold the motor in place to stabilize the motor, but that alone will NOT fix the problem. The two grommet method helps, but it will not achieve the potential of this fix.
All motor-borne rumble disappeared, microdetails now appeared, soundstage improved with greater clarity, warmth moved to more neutral, excessive woofer movement disappeared. In my system it worked, but I'm disappointed in Pro-ject.
Make the fix and keep your table.