I will state up front that I was not an owner of these.
However, I was the friend of a fellow audiophile who did own them for a couple of years, and I got to hear them, a lot.
As far as tube amps go, this is about as good as it gets, until you get up to the VTL ref. 750s.
(My friend tried the following tube amps, (before he settled on the 450's): Kora Cosmos, BAT 150s, Manley Neoclassic 250s).
They image and soundstage great.
They have very good bass response, (for a tube amp).
They have a great mid-range and a very good treble response.
However, they are not overly tubey or euphonic.
Their one down side is that they run hot, Hot, HOT!
(That and the fact that you have to bias them and replace a fair amount of tubes when needed.)
Used they run around $4,500-$5,000, and for that money, it is well spent, IMHO.
However, that being said, if you can afford half again to double that much,($7,000-$10,000) you can get what both my friend, and ultimately myself, consider a better amp. The 220 wpc hybrid Lamm M2.2s (or their predecessor, the 200 wpc M2.1s), are better in pretty much all aspects. (After I heard my friends new M2.2s in his system, I went out and bought a used pair of M2.1s for my system.) They image and soundstage as well, if not better. They have slightly better treble response, and quite a bit better bass response. (Maybe not Krell bass, but it is deep, solid and tight, and it integrates well with the rest of the frequency response, which I am not sure the Krell's do - IMHO anyway.) And, even though they are rated at half the MB 450's power, if you are running less efficient speakers, (say 4 ohm or less) than they actually seem as powerful, or even slightly more, than the 450s.
The only cavaet is that they too run hot, Hot. (But not HOT! :-) )
My two cents worth.
Good Luck in your search.