The presonus (MP20) is generally considered a pretty decent mic pre, but a little over-priced considering there are others for less money that sound just about as good (dmp3), and others for similar money that sound better (RNP). It is pretty impressive, though, that at it's price point you get a two-channel Class A with transformer input.
I wouldn't even touch their Blue Tube thingies. Pure junk.
The Bellari is an interesting piece of work. It's the only budget tube mic pre that
really is a tube mic pre, and sounds like a tube mic pre. It's not a starved plate / hybrid deal. There is a catch, though -- it's not necessarily a really
good-sounding tube mic pre.
It's slow-sounding, for lack of a better word, and a tad noisy. It's surprizingly cleaner than you'd think it would be (considering it's pedigree) -- but it will impart a certain old-school, low-fi type sound as you start stacking the tracks. Not necessarily a bad thing; a lot of early-90's grunge and some of today's punk and indie band's go for that kind of thing.
Personally, I think it's cool as hell to have as sort of a special effect. I use it all the time on aggressive-sounding rappers, by the way. I got this big, big dude who comes in now and then with a pretty intimidating kind of voice. The Bellari actually gives his voice this old-school "fuzz" thing that reminds me a little like a (very) poor man's distressor-type compression. Weird and hard to describe; you'd have to hear it.
Keep in mind -- I did have the Bellari modded quite generously -- the capacitors were replaced . . . the input transformer replaced with a Jensen, and the stock tube (recently) replaced with a T'ung S'ol, etc.