Re: O.P's thump:
It's probably either a dirty switch which needs a clean with servisol or the 2 caps across the switch need replacing. My IIX900 has a couple of 22 or 33nf (0.033uf) polyesters (MKT, orange drop type), but X2's would be better. Need to be 400v dc as it's for mains AC.
Re: SB11's OL motor:
The best thing to do with that heavy baseboard is to remove it altogether!
That's what Systemdek did with the 900's and 2X2's anyway. It was cheaper to make and sounded better (most people took 'em off anyway). The bit of hardboard under the later models was just to cover the mains wiring for safety.
Generally the order of play was:
1) remove base
2) fit better armboard (on IIX's as 900's were alloy anyway)
3) Fit acrylic platter.
Damping does help, but only if it's LIGHT, like cork or thin sorbethane/deflex fitted under the alloy sub-chassis. Heavy damping defeats the object of the design and won't improve things.
One thing I did was to replace the original motor with a low vibration Premotec one from a rega which is much smoother and quieter. Used with a decent thrust-bearing (available from at least 2 E&*y sellers), will be not far shy for a lot less outlay!
I just wish I could get a new set of springs as mine have seen better days and trying to get a response from Audio Note is like waking the dead