Yes, I do think the top plate tightness to the plinth makes a significant difference and vibration at the motor corner is certainly not a good thing. Linn's solution of bowing the top plate is elegant in a sense but it isn't as effective as it could be. I've recently had a chance to play around with a Khan top plate and that certainly seems to give a more solid and consistent contact to the plinth (it bolts to it at a number of points).
You can only really tell how well the top plate is fitted during assembly of the deck. So to check it properly you'd need to strip the deck down (i.e., remove the sub chassis) and then basically tap around the top plate edges to determine if it is solid with the plinth in all areas, particularly the corners. Then of course you're left with the job of re setting the deck which if not done properly could you leave you in a worse place overall.
The corner bolt does help in terms of ensuring that the motor corner area is well connected to the plinth so that motor vibrations are better damped, but if you have a well fitted top plate then it doesn't make much difference. What it did do was add more consistency to deck set up. You can do a mod by either drilling a hole in the corner area and the corner brace and fitting a long M5 bolt, or by picking up a S/H top plate with the corner bolt already brazed on, there are quite a few of these about now.
You definitely want to check for rattles when it's stripped down. Too tight kills the sound!
Oh, am I!
No. Prob .
My LP12 has a well-fitted top-plate, to which I applied the bolt-mod as described. I certainly found that anything thing more than just nipped up to provide a light tension absolutely killed the Linn groove.You definitely want to check for rattles when it's stripped down. Too tight kills the sound!
Been there done (and undone) the bolt mod.
I found that taking time to get the top plate formed correctly is all that's required to make the motor corner (and switch corner) rattle free. The flaw inherent in the bolt mod is that it introduces one extra tuneable component into the (arguably) already too tweaky LP12. Aftermarket top plates with multiple 'bolts' are an extension of that - for good or bad.
Mr Tibbs