When it comes to various speakers that are available. The most important factor is the enclosure the speaker is mounted in. The Mark III that I used to own had the smallest footprint for a speaker enclosure but sounded full and complete. More than likely due to the EVM12L Black Shadow. I have tried a few 412 cabinets with the combo amp. The Recto Cab with V30 was okay, I also had a Crate with unknown speakers in it. When compared to the Mark IV that had a MC90, I preferred the tone of the Mark IV over the Mark III. However, the MC90 just did not deliver the same performance as the EVM black shadow when the volume was kicked up beyond bedroom levels. What killed it for me was the breakup of the MC90 and the EVM did not deviate from providing that what goes in comes out, only louder. Then again you have to consider the voice of the amp as they are tuned differently. I had my Rectifier cabinet for 14 years before I decided to change the paper for something different than the V30. Before I did this, I first installed an EV into the Mark IV (the MKIV is the closest combo I have to the Mark V). The EV made a huge difference, if it fit properly in the amp it would still be in there but opted to re-cone the Recto 412 cab with EVM12L. Before the change, the Mark V head still delivered but tone of the V30 was not what I liked. The big factor is the size of the enclosure. Cabinet was the larger one (standard, not traditional). It was not until I bought an RA100 head that I realized the EV is not best for all amps. The RA100 sounds stellar with the V30.
When it comes to the combo, the Mark V is the exact same size as the Mark IV. I almost considered getting another Mark V but in combo form. As I was preparing for building a new 412 cabinet I had considered a few alternate speakers. WGS Black Bird sounded incredible with the Mark V (mounted on in the Mark IV enclosure). By far that had the best tone and character for that amp. The only issue is that the WGS (rated for 100W) may be over-rated and is probably a peak rating and not continuous rating. The Jensen Black Bird is similar (WGS modeled their version from the Jensen) has a much higher rating 130W peak / 100W continuous. If I had a Mark V combo, I would go with the Jensen. That would be a direct drop in without mods. I did hear some sound clips of the KING SNAKE, reminded me of my old Mark III (tone wise).
In most respects the MC90 sounds good, however the one I had would flub or have too much break up as the volume increased. I had the same issues with the V30 in the 412 cabinet. Breakup may be related to the size of voice coil, most 2.25 to 2.5 inch coils will support the full frequency range but will be low frequency dominant at low volume. The smaller voice coil of the MC90 and the V30 seem to enhance the mids and higher frequencies and fall short in the low end as the amplitude of signal is increased. There is much more to add to this but I do not want to write a novel. I am already getting too far into the weeds as it is. Sorry for not being high level and to the point.
Just a note: My Mark IV ( non-modified amp, version b ) some how may be delivering more power than the Mark V head. I do know the amp is almost at 100% with a setting of 3.5 on the master. While I had the MC90 in the amp, I could not go beyond 2 or the speaker would break up severely. I settled on a Fane Studio 12L for the Mark IV. I have been able to push the envelope with that speaker and take the Master volume all the way up. Settings above 4 just seem to compress the sound (does not get louder) so the tubes must be operating in saturation. The Fane is very close to the EV classic. I wish I could get more but do not want to pay the high shipping costs.