I used a (discontinued) Randall RT2/50 when I had my M4, but if I were to go that route again, I'd look at the ones below
Peavey Classic 60/60 - has 6L6s, if I'm not mistaken
Peavey Classic 50/50 - uses EL84s
Mesa 20/20
Carvin T100
I've heard the Peaveys are workhorses (as always), and I'd probably pick one of those up first. No real complaints about them and they can be had quite cheaply on the used market.
I've heard good things about the Mesa, and to not worry about it being underpowered due to its size (1U).
I've considered the Carvin before; some love it, some are just 'meh' about it. I've heard that it is 'flat' (i.e. no real character of its own), but that's not necessarily a bad thing.
All that being said, there's supposedly a 1U power amp coming out with the new Synergy stuff.