I also asked Nik. I was hoping to get some answers from real world players but thats cool. Here is his answer for anyone who is interested:

Richard
 
It's best to divide them -
 
OTS and S&M are 80s Dumble ODS.
 
HRM series are 90s.
 
 
The HRM main difference is that its got a post OD tonestack inside the amp for the OD channel.
 
Then, the master isnt shared between the channels. MV works for clean only, which makes it easier to preset volumes, perhaps, for live.
 
There are other feature differences as well, incl. actual Deep switch.
 
Sound wise -
 
The HRM is much gainier and tighter on the OD of all. You can definitely tweak this to match what you want.
 
The Bluesmaster has excellent cleans, but the OD isnt gainy but more articulate.
 
The OTS is perhaps the most typical "dumble tone" - dark, fat type. and the 80s are more organic and more open than the 90s.
 
The S&M is very open and brighter than above. Its more open/organic than even the OTS.
 
 
Hope this helps!
 
Thanks!

Nik

From the horses mouth too..!
That bit of information is very enlightening.

Thanks for posting that...  Cheers.