Mastersound Compact 845 preamp upgrade

Do you mean having what you have modified or upgrading to something you might like more from a musical standpoint?  I will tell you, as far as modifying goes, I would not let anyone touch anything unless the person doing the work has a set modification for the unit that has proven itself to be an improvement - such as Modwright's mods to the Oppo DVD players.  Just haphazardly replacing parts with more expensive parts is nothing but a crapshoot, IMO.  You need to know what you're going to end up with.  If you mean upgrading to other components, you would have to list your system, the type of music you listen to, the volume you want to achieve and what your sonic priorities are, i.e., midrange naturalness, concert volume, huge, powerful bass.  Whatever. 

No, no, no. My upgrade would be to put Mastersound preamp in my setup. Now I have only Compact 845 integrated amplifier. It has direct input, and with that input conected, becomes power amplifier.

I wonder, what would preamp do to my system...

A good preamp can make a huge difference in the sound, but unfortunately, it's impossible to tell whether these differences will be meaningful to you.  Tell you the truth, I had a Mastersound DueTrente a few years ago and it was a nice enough amp, but just sounded "colorless" for lack of a better word.  I guess you could also say it lacked harmonic bloom or whatever tube lovers like about tube amps, admit it or not.  I don't know what the 845 sounds like, but if you're not particularly happy with it, maybe you should just sell it and move on.  I like my Viva much more than the Mastersound, although it was admittedly twice the price.