I have done a lot of equipment combinations and the idea that equipment from the same brand is "designed to work together" is really just a marketing ploy, in my opinion. That being said, if you want to keep the McIntosh aesthetics to match your amp, by all means get a McIntosh MX processor.
I generally recommend items based on the type of sonic signature the person is looking for. In my experience, the McIntosh equipment is very full bodied, but laid back in a sense.
Krell Foundation is going to be very forward with a lot of attack and resolution. It could be a good match to compensate for the laid back character of the McIntosh amp.
Marantz is very good stuff, especially the AV8802 or AV8805. Keep in mind that they have a very large sales and distribution network and will be able to sell the item for less and still be able to get a return on investment. You will get "more per dollar" on the Marantz than you would for a Krell or McIntosh. It's hard to say what would ultimately sound the best (Krell / Marantz / McIntosh). The McIntosh does have beautiful front panels, however.
Anthem is well liked. However, I think it's slightly bright sounding. It does use switching power supplies.