I don't know your particular unit, but you mention the possibility of replacing it with a Mastervolt Combi, which is what I have fitted. I can therefore confirm that the feature you are worried about is selectable (or otherwise) in the MV. They call it Mains Support.
I'm really quite impressed with mine, which is a 12/2500-100. It's really very controllable, but in order to achieve that is also quite complex. However, it is a clever bit of kit, will support the correct bulk/absorption/float voltages for all battery types and will also allow you to equalise your batteries at 15.5.
The manual with all the possible settings is downloadable from MV's website. I'm a Chartered Engineer and I confess I had to give it a few reads before I was sure I had everything as I wanted it. If you read it and scratch your head I'd be happy to interpret if I can!
ETA: It's never, ever buzzed at me, and I turn it off when I don't need 230v - unless I'm on shore supply and then I leave it to do its stuff.