What the *#$@ happened??

You could ask Clearaudio for the advice. As for tuning as opposed to just absorbing, I will be the first one to support this approach. After all I have Michael Green free resonance speakers. You can easily overwhelm them with very loud music, but within their operating range they sound very natural.
Still I am puzzled by the collapse of the soundstage depth that you experience.
Also, turntable is not a cd player, you cannot really extrapolate.

"Technically Symposium is better than wood."

Technically it's better than peanut butter, too.  Or say a can of Spam.

Hi Shawn,  Are you judging based on aural experience or based on sound reproduction?  You stated that with the new plinth you "hear more detail in the music, subtle nuances are more prevalent, better instrument separation, especially in complex arrangements."  That does not happen by a miracle.

The stylus must track the record groove more precisely to retrieve this sound information.  The stylus cannot selectively track the more complex passages and fail at the simpler undulations.

I suspect that you are listening to studio recordings rather than live recordings of acoustic instruments and voices.  Studio music sounds flat when heard as originally recorded.  Conversely, acoustic instruments have timber and when recorded in a hall (like classical music), they have an ambient sound.  Nothing is added artificially, like reverberation in studio-recorded music.

Before your new plinth, you believed your listening experience was like hearing a "live" performance.  But what you really were hearing is echo of the energy from the stylus tracing the groove and the energy resonating back into the stylus rather than being absorbed in the massive plinth.  Thus, it gave it a fuller sound, yet with less sonic detail, as you stated.

I suggest you listen to your new plinth for a while and experience what you have been missing.

Maple is the most popular, I never heard anyone use ebony or ironwood, as an example.

My plinth, platter and armboard are made of cocobolo, which is denser than ebony and similar in density to ironwood. The plinth and platter are also loaded with multiple chambers of lead shot. The maker (sadly, no longer in the TT business) experimented with and sold TT’s made from many hardwoods, including ebony, over the course of a 5 year development project. His findings were generally that, the denser the wood, the better the sonics... for the reasons Larryi explained. I heard several iterations, owned three, and completely agree with those findings.

Maple was tried early in the development and quickly discarded as less than satisfactory when compared to denser (though admittedly more costly) options.

Larryi’s post was astute. Listen closely and try to distinguish between sound that you "like" vs. sound that is more revealing of the actual performance as recorded in the grooves. On a sufficiently revealing rig, the artificial aspects of studio recordings become apparent. A good TT reproduces the truth, whether we like it or not, and it sounds like your tweak may have gotten you a step closer.