rvshultz:
I currently own a MkIV widebody combo (purchased in '95) and a MkV combo (purchased in 2009).
I previously owned a RK head (version 1) and matching 212 cab.
I also previously owned a LSC 112 (early version without the 10W option).
I have never owned or even had the opportunity to play through a MkIII.
There is no question that all of these are great amps and have their individual niche. Yes, I am presuming on the MkIII, but doubt I will get any arguments here!
However, based on your thinking of downsizing to one amp that is most likely capable of providing sounds similar to your LSC and MkIII (and will also offer a lot of other very nice sounds)......is the MkV and here is my reasoning based on the modes offered in the V:
Ch1
Clean Mode: Super clean with max headroom (think MkIV clean in a fairly neutral setting)
Fat Mode: Very nice clean with more girth (LSC)
Tweed Mode: Think nice low to mid gain breakup that reacts well to guitar volume knob
Ch2
Edge Mode: Think super urgent brightly voiced clean with low to mid breakup
Crunch Mode: Think nice high gain crunch sound, but can also be used for nice lead sounds
MkI Mode: Dark voice lead sounds with lots of sustain (LSC Ch2 can be set kinda like it)
Ch3
IIC+ Mode: Can be set similar to an actual IIC+ and offers many sounds of its own
IV Mode: Can be set similar to an actual IV and offers many sounds of its own.
Extreme Mode: Can be set similar to an actual IV with many, many other sounds of its own.
The MkV also offers GEQ, multi power settings, inidividual reverb level per channel and solo boost to name a few. The footswitch is very user friendly in a live situation.
MkIV: While this is also a great sounding amp....its features are more limited...and you would have a difficult time obtaining a LSC clean with this amp.
Road King: This amp is loaded with features and does offer 4 dedicated channels, but the lead channels are based on the recto.....and you would be hard pressed to obtain a MkIII lead sound using this amp.
Folks, these are just my opinions based on my experience with these great amps.
I hope this helps.