TT mats

bdp24, That's the crux of the matter, when it comes to platter mats, the transmission, or lack thereof, of energy between the LP and the platter.  Do you want that or don't you want that?  Each of us has to answer that question "by ear". If you want that, you use a platter mat that is similar in material construction to an LP, then you clamp the LP, and you might also use a peripheral ring and/or vacuum hold-down, if the TT permits. If you don't want that, use a Resomat or one of those with cork dots on it, no weight or clamp, no peripheral ring.  Most tune by ear using a combination of energy absorbing and energy reflecting techniques.

Holy Moly!
I never realised there was nearly as much science in the platter mat as in the whole TT itself!
Now I am going to have to experiment!
Been using an Achromat 3mm for a while and cant say  as ever thought to experiment but.....
As an aside I could only find a couple of examples online of the SAEC SS-300 and these were in Japan and about $420.   Eek!

"Micro Seiki CU-500 is super heavy (2.7kg) pure copper mat, very rare and very expensive (over $1200), better than SAEC SS-300, but not for every turntable. I use it on SP-10mkII and they are looks like they are made for each other. I remember when i replaced stock rubber mat, the effect was huge! It was the biggest upgrade for my SP-10mkII. CU-500 and thinner, lighter and cheaper CU-180 does not fits the Luxman PD-444 well, so it is always important to measure the platter and mat before buying any of them.

I'm fine to put the records on the metal mat, i think the engineers of Micro Seiki knows well what they are doing. The platters of Micro turntables are copper. I don't have the ST-20 clamp, it's extremely rare, but i use ST-10 instead. I think the ST-20 was made for CU-500 mat."

Slight correction, the Micro Seiki CU-500 and CU-180 are (were) made from gunmetal copper alloy.   

Only a handful have ever made one from pure Copper, like the Artisan Fidelity Pure Copper Universal Platter Mat (3.6kgs.).  Robusto used to make one, but had to switch to gunmetal copper alloy they said to reduce production costs.

The Boston Audio Design mats are decent.

I mentioned above that I was about to experiment with a custom-made copper mat for my L07D.  Well, I now have done the experiment, and copper is a winner over the OEM stainless steel platter sheet.  I was very surprised at the degree to which the copper sounds better.  The background is as follows:  On the "L07D Owners" website, hosted by Howard Stearn (not THAT Howard Stern), there is a suggestion that one might wish to do something to block EMI from the motor that might affect the cartridge.  The standard stainless steel mat ought to be at least somewhat effective in doing that, but Dr. Stearn and others suggested that a little more could be done.  I originally used a piece of TI Shield cut in the shape of an LP, inserted between the platter and the platter sheet so it was invisible.  This did make an audible improvement.  However, when I tried to use a very low output MC on my L07D, there was a problem.  The magnetic rotor of the L07D motor was inducing magnetism in the TI Shield, which is ferrous.  Before I noticed this, I nearly crushed the suspension of my Ortofon MC2000.  So I had to ditch the TI Shield for MC cartridges.  This resulted in a noticeable degradation of the sound, already suggesting that one must block EMI somehow.  Other L07D owners use ERS cloth between the platter and the platter sheet, but I was reluctant to do that because it would mechanically decouple the two components and is likely to affect sonics.  (For all I know, it could be fine to use ERS, too.)  I then hit upon the idea to use a copper platter sheet.  Copper is much more effective at blocking EMI than stainless steel.  I sent photos of the original, and the dimensions and desired weight, to a machinist, Colby Lamb, in Oregon, and Colby made my copper platter sheet which exactly matches the original SS platter sheet in appearance except for being about one pound heavier, and by the way is gorgeous to look at.

With the copper mat, a faint gray-ish coloration that I had noticed since removing the TI Shield is now gone.  The music is much more "open" sounding and does not lie on the speakers, as it had done when I was using no extra shielding at all.  There is a lot more "bloom" and spaciousness.  So, my friends, block your EMI, especially if you are using a coreless motor; the field of a Dual-type coreless motor is directed vertically toward the bottom of the platter. (Both the L07D and the TT101 and many other coreless TT motors are copies of the original Dual coreless motor.) It's possible that an iron core motor has much less tendency to radiate EMI that might affect sonics, because its field is in the horizontal plane rather than the vertical.  This is purely a guess on my part.