Are u sure they are graphite?
Graphite tends to resonate and ring when it vibrates - I always use slate - it is formed in a different way to granite and doesnt resonate. I have floorstanders with spikes, i used to just have them through the carpet - however I bought 2 evenly sized/same thickness slate patio tiles to stand them on now - result crisper bass more punch less vibration through the house (isolated) cost £10
A friend had some made at a stonemasons - polished smooth and bullnosed £200.
Anyway I wouldnt include the tiles especially if you have doubled them up - also tiles are clay and not very dense therefore they will vibrate. It is all a matter of taste and opinion - if you are happy then go with it, someone on here said always try experimenting with your set up - something that I hadn't done for a few years - although small I decided to position my speakers flat on instead of toed in and even this has made a difference to the fact that i now prefer the sound