Replacing cork platter mat with acrylic mat

After reading some reviews touting the benefits of acrylic platter mats, I ordered one, out of curiosity.

It arrived yesterday and I sat down for an A/B listening session. Played "Car on a Hill" from Joni Mitchell's "Court and Spark". Beautiful, gorgeous, lush sound.

Then replaced my cork mat with the acrylic mat. Played the same track -- WTF? Terrible, congested sound, the bass became timid and all the sparkle was gone! The sound was wimpy, claustrophobic.

Placed the cork mat back on, played the same track -- aaahhhh, what a relief! Gorgeous sound is back in its full glory.

So what gives? How is it possible that such minor change results in such shocking degradation in sound?