Re: Opinions on the Mesa Boogie Express plus 5:50 1x12 combo
I have one (although I converted it to a head and 1x12 external cab).
The gain channel doesn't sound like a rectifier's red channel, but more like the vintage (orange) channel. Grittier, less scooped, less bass. (I have a dual recto also). The clean channel 1 has a higher gain switch "pushed", and it will let you get more Recto-like crunch but not at comparable gain levels.
I like mine a lot. The cleans on the 25watt setting are really nice. The dirt channel isn't perfect but I have grown to like it after messing with it. It has plenty of gain for me, sounds great for solos; for rhythm it sounds more stoner-ey and grungy than like super crunchy 80s thrash metal.
It is a very versatile amp. The power scaling is useful, the EQ strip is cool, and the sprin reverb sounds good.
One thing I would add is that this amp really pairs well with the Mesa C90 speakers that the combo comes with. I used it with a V30 based 4x12 and didn't like the results, too squishy and buzzy for me. I ended up getting a C90 based 4x12 and liked the results better.
I think it's a great amp, extremely useful and versatile, but it is different from a dual recto.
I don't have a Mk V, but my understanding is that while there are some similarities such as the 5 band EQ and layout/ergonomics, etc. they are tonally different amps. Boogie Bill must weigh in also, he has a lot of experience with Mesa (of course) and I believe he has a Mk V.