Loricraft Garrard 301 to Dobbins Sp10 Mk3 - What can I expect ?

There are families of DD turntables, just as there are families of belt-drive and idler-drive types.  You auditioned only one "brand": Technics.  First of all, I find it hard to believe that you didn't hear substantial differences between the 1200G and the other two and between the Mk3-based Dobbins turntable and the Mk2 with no plinth.  The 1200G uses a coreless motor, which makes it fundamentally different from its forebears.  The Mk3 in any iteration should blow away the Mk2, let alone a Mk2 with no plinth.  I easily heard big differences between Mk2 and Mk3 in similar slate plinths, in my system. However, it's possible that all 3 take the Technics approach to servo control, which is very tight.  I don't like to think so, but perhaps you are "hearing" that tight control of platter speed that results.  There are other DD's with coreless motors and "looser" servo feedback mechanisms, like the Kenwood and maybe the Victor TT101 (which would have to be re-plinthed or the plinth modified) that you might find more like your 301, but better.  And there's also the Luxman PD444 and the Yamaha GT2000X.  So, you have DD's with iron core motors vs coreless motors, DD's with heavy vs light platters, and DD's with varying tightness of the servo control mechanism.  Each of these design choices makes a difference to SQ.  Not to mention the effects of plinth, tonearm, cartridge.  Generalizations are hazardous, because they make you stop thinking.

One important difference I missed out is, from 1200G to Sp10 Mk2 there was a marked increase in slam factor. 

Interesting. I run 2 different AudioGrail 401s. One is in a slate plinth with new 3rd party platter and idler. The other with original platter and new idler in a birch ply plinth with walnut top. Both sound great. Extremly musical and every record sounds great. The slate version is faster and to my ears, more neutral and transparent. The benefit of speed is critical in preserving the clarity in a way that mere detail does not. I heard the new SP tables in New York and prefer my Garrards. Obviously, there were many other factors but what was missing was a musical rightness that you have expressed.

one thing that I found that really made the 1200G sound better was to adjust the torque to its lowest setting. The factory setting was not right and made the player sound as you described.  I find the 1200G to have plenty of slam.  I actually like it better than the sp10 mk 2. 

I tried the 1200G with lower torque, default torque and also higher torque. The sound did change a bit. The thing is when you want to “fine tune” these things matter. When you are looking for a more fundamental change in sound these things don’t help or doesn’t matter. I am not saying someone liking the Technics is less critical. I think it is all about how the ears have gotten trained over the period. After listening to the flow and uncompressed dynamics of idler the DD sounded too controlled. I can imagine exactly the opposite if I was a long term DD user and listening to idlers. I would probably complain about the noise, control and less organized presentation of idler.