Plinths for Monitor Audio Silver 8


I purchased a pair of Monitor Audio Silver 8's a couple of months ago and they sound great.
I have what is definitely a newbie question.
They came fitted with nice heavy metal plinths and then in separate boxes are two more plinths.
The ones on the speakers are flat and raise the speaker a couple of inches from the floor. The extra ones are slightly rounded and would seem to raise the speaker up just a bit more.
What is the reason for the two styles and how do you determine what is best for a particular use.
I am using the soft silicone feet on a hardwood floor.
Thanks
128x128granburyspring
Yes, I bought them used at a resale shop for $299. They were sitting in a corner gathering dust. 
I did get the separate boxes with 2 additional metal plinths in them with screw on spikes.
The speakers themselves are fitted with metal plinths on the soft silicone screw in feet.

They sound incredible as is. I was just curious as to why there were 2 sets. Perhaps the original owner had two sets of speakers at some point.
My silver 8s  were purchased new from (I think) Music Direct and came with wooden plinths with a set of spikes and a set of plastic feet with rubber pads; their use depends on the floor surface. I did not get any metal plinths, nor did I get them for $299 (lucky you!!!). 

Tom