So I replaced the original Garrard 401 mat with an EP

Did some back and forth and can confirm the result of removing the mat and replacing it with a 10" record:
1. Warmth - a slightly yellow-brown 'coloration' has been removed
2. Heaviness - with mat, it sounds like the musicians are behind a curtain and on a smaller stage
3. Brighter - As @pani said, it is definitely brighter - but much clearer. Rim of drum and plucked bass distinct, clear and fixed in space.
4. More air, depth, and dark space between instruments instead of a wall of warm sound - less lush. plucked strings snap into space in a startling manner. 
5. Increased pace - I had to test speed with strobe as I was sure it was running faster without heavier mat - it was exact.
6. Sounds louder - confirmed by others present.
Notes:
a. I can understand bright recordings being irritating but the improvement is too much to ignore.
b. I am convinced the mat damps and veils the sound. It's like going from Mullard to Telefunken.
c. Surface noise frequency spectrum shifted up slightly.

I replaced the original mat on my Garrard 301 with the SPEC+ Analog Disc Sheet (AP-UD1)

A nice improvement to the clarity and resolution.

Same... I replaced the original mat of my 301 with a SPEC+ mat and love the improvement.

Thanks for the recommendation. Reading about the Spec+ gave me an idea.
I just swapped the 10" vinyl record (Daughter - His Young Heart EP) for a 10" shellac 78.
Wow! All the improvements listed above even more pronounced:
1. Lower surface noise
2. Improved depth
3. Hearing further into recording
4. Better focus and more neutral.

noromance,

I played around with several mats and found the original Garrard mat on my 301 sounded best. Same with my Dual 1229. I do like the original Oracle Platter mat and will use it occasionally.  A audio buddy of mine recently bought a solid copper mat and I was really impressed by it.

I also like your idea of using a 10Ep. I have some 78's and might give that a try as they are very rigid and flat.  It seems that the softer the mat the warmer the sound. The harder the more dynamic. 

N.