Do editors choose reviewers based on the quality of manuscripts?
Editors try to maintain a good status of papers published in "their" journal. There is no obvious point in using different standards for different papers and I am sure that such differences are not concious decisions if they appear. If an editor receives a paper that is outside of their comfort zone, it is of course possible they are not aware of possible good reviewers and may make seemingly odd choices. How common this is will be difficult to assess but that it occurs is certain.

In the specific of receiving something of breakthrough character, the procedure should normally be the same. However, an editor always has the choice of adding as many reviewers as they see fit. If a controversial paper appears, it will likely receive quite varying reviews and adding additional reviewers may be useful to get a sense of the quality of the work. This does not necessarily mean an editor will go for specific names but I can see that it may be the case that one would go for prominent names for such a paper. 

In my experience as editor and author (and reviewer) I often get the sense that good science receives far harsher reviews than poorer. I do not think this is because the good stuff are assigned to "better" scientists, I actually think it is because a good paper is easier to fault be cause it is more transparent than poor papers which are muddled.

So, in short, there are many aspects to how papers are run through a review process. Choice of reviewers is not generally done after quality of the paper although it can happen. In the end a journal, through its editors, want to retain its status and letting poor papers through, does not serve such a purpose.