I've only had the Magellan and Mesa together for a couple of days. And man, both these heads are, in a word, totally awesome. OK. That was two words. But they are. Totally awesome, that is. It really is a case of apples-n-oranges.
I brought both to a small rehearsal/jam with a drummer that is extremely mature, controlled and conscious of volume (but never loses any intensity) and a guitarist/blue-eyed-soul singer. Good time! Played my modded 3 pickup Scheff (Hanson Vintage J's and Vintage P and a Hanson/Lakland preamp) that is muddy and a Barefaced One10. The trick to the Scheff is cutting bass. Cut some healthy amounts of bass with a good HPF or Bass control and its tone heaven.
I was surprised that the Scheff sounded great and clean with just a hint of muddiness with the Magellan's controls set flat. I used Contour B set a 9:00 to clear up any mud and that was it. I could toggle between the Clean and Dirty channels at will and was very pleased with the tone of both. Really solid, punchy, clear tone. VERY easy to dial in two great useable and different tones. Overall, maybe a little sterile or lacking "character", for lack of a better word. But that's purely subjective. Or is it objective?? I can never remember.
The D800 was also easy to dial in. I set Gain to noon and set the controls flat and the Scheff was too muddy, like I would assume it would be. Set the Bass to 9:00 (see my comments above on my Scheff) and dialed the Treble back just a hair and that was it. That fine line between thick and punchy and pillow and grit. The D800 really cut through without being clanky. Nice! I would NOT call the D800 sterile. Lotsa "character".
I also tried both heads as just "amps" with my Grace Felix and my Full Circle equipped upright. To my ears both had the same volume. The Magellan was clearer and dead quiet. The D800 had some noise to it and had a little more bottom. But man, these differences were pretty subtle. I would give a slight edge to the Magellan for use as a power amp.
The D800 wins in the ohms department. The Magellan is the winner in the clean department, for sure. Also wins in the two-tone department. But I would give a slight edge to the D800 in the character department. The D800 had more...Hmmm...What do I call it? Not grit. Not punch. Well, it had more "character". More soul. I felt like the D800 had something to prove. Like it was part of a tradition. Sounds silly, I know. But that's how I see it.
I came into this assuming I'd like the Magellan more. But I don't. I like them equally. It really is apples-n-oranges. Both are roughly the same size and weight, roughly the same price, roughly the same volume, and both will run below 4 ohms. Both seem very well built, are good looking and easy to use.
@Jeff Genzler and @agedhorse should be proud that they've set a new standard.
And I've heard that Mini Mega ain't too shabby, neither!