OK, so your Technics sounds better to you than did your "Sota, Rega, Linn, AR, Thorens, Dual". Each of those brands comes in several flavors (except for the AR). If you owned the low end of each line, your results with the SL1200 are not at all surprising or remarkable, IMO.
Now as to the "steel plinth cover", I am a bit confused. Is this kind of a "cap" that sits over the plinth so that the platter and tonearm poke through it, or are you talking about what is in effect a dust cover?
Finally, it's a wonder the platter can still rotate with all that added mass from your aftermarket platter mat and record weight. Theory says that adding so much mass might compromise the proper operation of the servo system, which was designed to cope with the inertial mass of the stock platter and mat. Also, the bearing is under extra stress, which may or may not be a longterm issue. But if the net result pleases you, it's all good. I do think that getting rid of the stock rubber mat (or in your case, covering it with another mat) is a good move.
Now as to the "steel plinth cover", I am a bit confused. Is this kind of a "cap" that sits over the plinth so that the platter and tonearm poke through it, or are you talking about what is in effect a dust cover?
Finally, it's a wonder the platter can still rotate with all that added mass from your aftermarket platter mat and record weight. Theory says that adding so much mass might compromise the proper operation of the servo system, which was designed to cope with the inertial mass of the stock platter and mat. Also, the bearing is under extra stress, which may or may not be a longterm issue. But if the net result pleases you, it's all good. I do think that getting rid of the stock rubber mat (or in your case, covering it with another mat) is a good move.