I have spent several hours A/Bing my MkIV Widebody 1x12 combo with my MkV 1x12 combo the last few days. I will start with a channel 3 comparison since that is what most people are requesting:
I am running both amps with stock MB preamp tubes and 4-6L6 power tubes. I am using a HSS guitar with pickup selected between single coil mid and HB bridge.
MkIV Widebody Combo Settings: Full Power, Pentode, Simulclass, Harmonics Selected
Channel 3
G-7 3/4 Pulled, T-7 3/4, B-3, M-6, D-6 Pulled, P-5 Pulled
MkV Combo Settings: Full Power, Pentode
Channel 3 - MkIV Mode, 90W
G-6, T-7 3/4, M-6, B-3, P-5
Note: The settings of my V are following the comparison guideline to a MkIV given in the user manual. The gain of the V is set to match the drive of the MkIV as recommended.
Truthfully, with these settings, my MkIV and MkV amps sound virtually identical. The V may be slightly brighter (not thinner), but that can easily be remedied (if it is problem for some) by switching out of bright to normal or with minor adjustment of the treble or presence controls. Personally, I prefer the sound of the V with the bright engaged for most applications. Kicks Butt!
The next test was to compare my MkIV with MkV Extreme mode. The only change was to push in the presence control of the IV, switch to Extreme Mode on the V and reduce volume of both amps! Once again the sounds of both amps were nearly identical. Both sound great.
I also compared both amps in triode mode, different power settings, engaging GEQ, etc...clearly, the V is able to cover the ground of the IV in channel 3 and in my opinion....beyond.
I also spent considerable time comparing both amps in Channel 1 and 2. While I am able to achieve great sounds out of the IV in channel 1 and even channel 2....it does not offer the excellent and useful mode choices that the V does in both of these channels. Secondly, I have always experienced a loud "pop" when switching from Rhy2 (set to high gain crunch) to Rhy1 on my MkIV. The MkIV shares the Mid and Bass tone controls for Channel 1 and 2. I don't know if that is the reason for the "pop" or not, but it has always existed with my IV. I have never experienced any "pop" whatsoever when switching between all channels of the V as long as I do not switch from 10W to either 45W or 90W (as mentioned in the manual). This is not a problem at all for me because I either footswitch with all channels in 10W or any combination of 45W / 90W.
Regarding, combo or head / cab.....I prefer the combo, but only because I don't like hauling cabs around. Both sound great.
Hope this helps...