So I replaced the original Garrard 401 mat with an EP

ORACLE GROOVE ISOLATOR especially when used as it was meant to be with the original DELPHI/PREMIERE platter, is a killer combination. Actually mat/platter becomes a very tight unit and this is the magic of early ORACLEs, with soft suspension to be exact. They totally went wrong with that hard mat from MK IV with stiffer suspension. To be honest, it sounded awful back in the day and I never looked back.

I have a GROOVE ISOLATOR on one of my DIY projects. Using vinyl record(s) as mat is a very good idea too IME.

Forgot about the thread. I did some more tests and am now staying with the 10" EP as the best compromise. 

Noromance.
Glad you linked back to this thread.
I found on my Victor tt-81 that a 10" EP gave the best results by far.
Very fast and dynamic, even better than the MyMat.... On that table.

On my 401 the MyMat comes out ahead.

Then a lead/ cork hybrid followed by the 10" EP.

Most interesting how results vary considerably between differing tables.

Hey Uberwaltz. Since I replaced the platter with the PAC 20mm oversized aluminium (it's UK made!) one, I prefer no mat. Rings like a bell -by design- until the record is placed on it. Sounds by far the most resolving.