Hey jazzyvee, my 2cents worth.. First, bottom cab off the floor (on wheels) will clean up bottom end (mud) as decouples the cab from the floor. Corner placement will add bass (corner loading). Top cab vertical, higher mids and top end will be a more narrow beam so as you found will open up more away from the stage, sideways (so MESA nameplates line up, I assume the way MESA designed/intended them) (although upside down should be similar since you have tweeter off LoL) will give wider dispersion of mids and tops, so clearer on stage and a wider dispersion out front. The top cab will also couple much more with the 4x10 and appear/sound like a 6x10 cab.
All the mids and tops from the bottom cab are going into your legs, and depending on stage height, and how they line up with peoples ears, they probably open up on the crowd but get sucked up by the front rows. (I'm assuming you are running full range into both cabs ?). If you are bi-amping with bottom end crossed over (at say ~180hz) to the bottom box and tops to the 2 x10 this is a little different. Typically this bi-amp approach works well with 15" on the bottom and either 2 x 10 or 4 x 10 on top. 4 x 10 on its own I'd put on a milk crate to get the tops up toward your ear level a bit.
But as you discovered, the best placement is what sounded the best in that place at that time. BTW it looks better IMO (though no pic :-( ) with the 2x10 on its side and the MESA logos in line.