I strongly recommend cats squirrel's website as a source of valuable information with regards to plinths, the science behind them, damping abilities of differing materials, combinations of materials etc...
It is quite an eye opener.
Mass loading the turntable is a way of getting its suspension / mountings resonant frequency down lower... - so the higher the mass the lower the resulting frequency (until you totally compress the springs / elastomers, and then suddenly there is no damping/isolation at all!).
This can be a good way to adjust things... if your current resonant frequency is coinciding with some environmental vibrations in your setup (eg: footfall) then shifting it down can sometimes resolve the problem.
Some plinth materials are also good dampers - but they have that effect at a specific range of frequencies - nothing is totally generic across all frequencies.
Layered wood products (Panzerholz, glued plywood layers, densified wood, bamboo timber) tend to be good dampers, but primarily at higher frequencies - not much effect in the subsonic range.
Plasticine is an effective damper in the 300Hz to 3kHz range (especially if sandwiched making a constrained layer) - but is a very easy to use, and reversible way of adding mass to adjust springing... so it can be used both for midrange frequency damping, and for low frequency resonance adjustment. (it is also dirt cheap!)
Hardwoods and any solid natural woods, tend to have specific damping frequencies and resonance frequencies (which will differ along the grain and against the grain) if the resonance frequency coincides with a frequency at which some other aspect of the tables design is a very effective damper - the combination can be very effective - otherwise it is likely to add that resonance frequency to the mix as a colouration.
When layered, subsequent layers are layed down with the fibres in differing directions - taking advantage of the differing damping/resonance linked to fibre direction - the glue between the layers also acts as a constrained layer damper (effectiveness depends on the type of glue...). As a rule this means that laminates of wood are far more effective than solid sheets of hardwood.
If you are serious about controlling resonances and improving SQ via plinth changes/tweaks - the starting point should be to measure the current resonances.
Your simplest measuring implement is your stylus, sitting either directly on the plinth or via a solid connector (in my case a spare counterweight did the job).
Then you can record the output using any computer and soundcard, and take a look at the frequency spectrum to see what comes up.
For mid to high frequencies, recording the plinth, while having the turntable in front of a set of speakers spouting high SPL pink noise should give you a fairly clear picture of what is going on.
For subsonics - It is more often related to isolation from the environement (stand/rack/feet) - and I tested it by recording, and walking around the room and house in a heavy stomping manner on my suspended wooden floors...
Measurement then allowed me to tweak and adjust my suspension and isolation to achieve a 20db reduction in LF "noise" at the plinth (problem zone was 2Hz to 10Hz) - the effect was very noticeable!
The plasticine in my plinth helped in terms of mass with the LF adjustment, but also helped in the midrange and highs which are now cleaner as well.
Other parts of my solution included mag-lev feet, concrete paver and sorbothane pads... I really would like to built a complete new plinth for my JVC, but I am not quite mentally up to the task yet... I have not (yet) worked out a design for the plinth that can house the electronics of the Servo arm, look good, and also provide improved functionality over the OEM plinth....
bye for now