Just spent an hour at the Mesa dealer. With a MR at low, medium and loud volume:
1x12 std recto: Good midrange grind, weak low end. Sounds anemic and overly-mid-heavy.
2x12 recto: Lots of midrange grind, enough low end to sound big and balance the mids, but very tight as well. Could stand to have a little more low end. Very "alive" sounding. Punchy.
1x12 3/4: TONS of low end. Nothing else. Very "dead" sounding, yet also "open" sounding. Could probably be improved by tweaking tone controls.
1x12 MR slant cab: TERRIBLE. Even worse than the std 1x12. It was sitting on top of the std 1x12, and is not nearly as deep.
4x10 (mine, from memory): Lots of low end, still pretty tight. Some midrange grind. Good top end. Very balanced across frequency.
In order of preference for recto tone:
2x12 recto
4x10
1x12 recto
1x12 3/4
Looks like I'm in the market for a 2x12 recto. Makes me want to dump the pair of 1x12s I have. Convenient, since I just bailed on my stereo setup and went mono.