I have owned many ARC preamps including the LS2B, but not the MK2. I also feel the preamp is the heart of the system. Much more important than the amp. I currently own a Ref 5se which I dearly love. My Ref 5se was a demo. Buying a demo or even used can save quite a bit of money. And it is already broken in. The LS28 is a wonderful preamp and would be a significant upgrade. Get the se version if at all possible. Speakers would also make a big difference.
My advice is to make a long term plan and work toward it. Upgrade one piece at a time. Maybe a preamp first and then speakers. You may not realize the full potential of the new preamp at first. But as you upgrade other pieces you will. And you will not feel the need to upgrade it for a long time.