I have not used the Symposium myself, but had done some research prior to purchasing my Parsifal Encores about a year ago. And had some discussion with Rick Schultz at RMAF just a few months back.
These are some thoughts/observations.
1/ The original granite platform between the monitor and bass unit should be changed out to something else that can dissipate the energy. The granite platform only transfers the energy from top to bottom and vice versa. The newer platform used on the Ovation has the solid aluminum plate + 4 gel-like pads that actually absorbs the energy and dissipated it. The Symposium platform does the same. I don't know which sounds better, but I ended up buying the one from Verity. A better solution, suggested by Schultz is to build a monitor stand completely separating the monitor unit from the bass unit.
2/ I have only tried the BDR cones (#4s) underneath the bass units. They sounded better than the original brass cones. I found this out the "painful" way. I had used the BDR cones every since I purchased these. I wanted to hear the brass cones one day and decided to put them on. They are a pain in the rear to install.unless you turn the bass unit upside down. I was trying to install the cones with speakers stanind upright. In the process of doing so, I put a dent to the front of one of the speakers. !#@#%$
I've seen photos of a pair of Parsifals using Symposium bases both in between the monitor/bass units and also underneath the bass unit. I believe it was taken at one of the audio shows.
What Schultz had suggested at RMAF is to elevate these speakers so that the tweeter is at or above the ear level. To do that, you've got to use more than just cones or platforms. He was in the process of getting a pair of racks built for his pair of Ovations, and had offered to build me a pair as well. I have a pair of SolidTech amp stands. I am planning on using them to jack up my speakers to see if Schultz is right.
FrankC