Hi Grifter,
I'm having the exact same problem that your going through. Just wish I'd come across this thread before I committed to the Sieg SC3. I'm no stranger to the Sieg, I had the C3 for 13 years before this one and it ran as smooth as silk. I got rid of it to make room for the SC3, sad loss. I've also got the Sieg SX3 mill and that's the same runs as smooth as silk.
The response from the supplier (the company from Leicester) seems to be the same as yours so reading between the lines I'm certain you got yours from the same place. I think the response from them is disgusting considering I've spent thousands with them over the years. Tried telling me that because of the higher torque of the brushless motor it can causes the vibration, never heard so much rubbish. The very nature of a motor that's not brushed contributes to it's superior smoothness. I purchased an er collet chuck at the same time which needed a 100mm backplate and they touched on that saying to maybe put bungs into the unused holes. I also got that the bearings may not be up to it so I have now ordered the taper bearings but I'm not convinced. I also got the we have run the glass test on our end and we get the same results here.
My vibration is annoying especially when trying to do fine detail turning but is nowhere as bad as yours. My vibration is also present just running the spindle nothing attached. To say these machines are acceptable is unacceptable in my eyes.
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What's the thing you haven't done, I'm very intrigued. Glad you didn't do the same as me and sold your old one.