TT mats

@analogluvr , good point. I use rather "lively" cartridges (London), and I’m thinking the Classic/London combo may sound a tad brash. London’s create a lot of mechanical energy, and a platter surface that is very hard (aluminum) may reflect it back rather than damp it a little. That’s the theory, anyway! The folks at the plastics place told me Delrin is harder than acrylic, for what it's worth. Also denser and heavier.

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.