I never owned or played through the IVA. I have heard them in recordings and thought they sounded better. I had a Mark IVB for 12 years. It was ok, hated R2 on all counts, except when I ran the amp through a 412 cab. Mine was a combo I could not stand. The MC90 speaker is not one I can admire.
When I bought my Mark IV, I had no clue it was a second revision. Just that it was much wider than the Mark III and easier to use. The Mark IVB has a 250pf cap coupling the grid to cathode on the last gain stage of the lead drive circuit. Made it boxy. The Mark IVA does not have this cap. Mark V90 has a 120pF cap coupling the grid to cathode on V4B (last gain stage of the lead drive circuit). Makes it boxy sounding. I had sold off the Mark IVB before I started experimenting with the Mark V90 circuits. Trying to figure out why it sounds like
(ice pick tones, could not dial in any treble or gain on CH3). Tweed and Edge sucked too. Now the amp sounds better than it did and can actually run the tweed or edge without the need of earplugs. Oh, wait, I ditched the Mesa branded JJ ECC83s tubes and used some older Mesa tubes from 1990.
As for what is better, IVA or IVB, it is up to user preference. What you want and like should be up to you to decide. If the Mark IVB is ideal to you, go for it. It is a great amp but can sound boxy if you get a combo. that is my only complaint about that amp. R2 sounded great through a decent 412, but through the MC90 it sounded crappy.