Preamp and amp combo match Martin Logan Sequel 2

Take a look at some anthem
also ive been a martin logan dealers for a while , and evertime they do a show they are always useing b&k to show off there stuff..

Look for a good 2 channel amp to start. Something that doubles down to 2ohm preffered. Older Levinson or Proceed HPA-2 ( what I am familliar with )
I have also heard some of the newer Classe powering Logans that worked well.
Then pick up a good 2 channel pre that will allow you to add a surround processor down the road IE Audio-research reff 1 or 2mkII. Get the 2 channel right first with your budget then expand later. You might find you are happy enought with 2 channel to not even bother with surround.

I forgot to add... use caution with the 3xx series Levinson as they have been known to have an expensive cap issue so keep that in mind when pricing a used one. Repair is expensive.