I'm finally on page one for my Sarah, which I ordered without ever hearing one.  I used to have a pair of Wavelength Cardinal X-1s, so I'm somewhat familiar with the sound of the 300B, but keep reading that the Sarah has all the midrange with the fidelity of the Zen.  

I've never heard the Zen either, but I own a pair of Alan Eaton 45s with National Union Globe 245 tubes.  I'm running them with a custom pair of Omega Super Alnico High Output XRS with integrated deep hemp subs.

What can I expect from the Sarah vs the 45 monoblocks that I'm using now.  I know it will have more power, but sonically what am I gaining/losing?

I agree with James that most of us had not heard a Decware amp before we ordered one--I have done that numerous times, my first leap of faith paid off so well I repeated and repeated and repeated. . . .

And like James I have never heard a 45 amp, and so can't make a comparison though I have lived with a SEWE300B for over two years now. I have heard two other 300B amps before buying the SEWE300B though, and can say this: in comparison the SEWE300B is less "romantic," but more dynamic and transparent. I don't find a pronounced midrange, but a clear and open one--though a lot can depend on the tube complement in use.

So I can't say much more than "I think you'll not be disappointed." And if you are, you can take advantage of the return policy, or I think you'll find you can sell the amp easily on the second hand market.