Guitar Center currently has the Little Mark 58R on sale for $510. I recently bought two. One went in my old (April 2014, early Indonesian) 121P Combo replacing the dead/unrepairable Combo II head. It was a super-easy swap: just peel off the feet (they came off a little too easily actually), tap with M4-0.7 threads the four sockets already provided in the top of the case, slide it in, replace the four affixing screws. No need to remove the top-panel. For Combo-use I really like that the efx loop and DI out are on the front panel.
While I had the amp on the bench I swapped-in and tried both a Eminence Deltalite II 2512 and an Eminence Basslite S2012. The Deltalite was not a sonic improvement ...and actually produced a bit of a mechanical "pop" when pushed... (and while it fit the hole, the o.d. of the Deltalite cast frame would have required that the 121P grill be trimmed to be re-used). The Basslite S2012 now in my 121P is a noticeable sonic upgrade and reduced the weight of my 121P combo from 25lbs 15oz to 23lbs 6oz. That's a weight reduction of 2lbs 7oz which is the exact weight difference between the Basslite 12 and the "Yellow Line 12LVY1-8" that it replaced.
I actually had taken my 121P combo to Guitar Center a month ago and tried a Little Mark IV head patched into it and was underwhelmed enough that I didn't see justifying spending the money. I know some folks have said they didn't notice much sonic difference between the Little Mark IV and the 58R. Having never owned a LMIV I'm probably not a good guage, but I think the 58R is a fine example of what "the Markbass thing" can be. I know my "hot-rodded" 121P is giving me a thrill. I even actually like the ABS case and it's slightly "softer" edges. ...Markbass may want to re-assess the "compatibility with ABS" regarding the adhesive on those feet though.