Hi! I think most Americans don't get an MA first. Foreigners, on the other hand, usually have an MA in order to make themselves more competitive. (as it is, adcoms compare foreigners to each other, and Americans to other Americans, because the backgrounds are usually very different.)
As far as the Subject Test is concerned, I have decided not to take it, since I'm applying for Comparative Literature programs, and most don't require it. I have checked out the test, however, and I think your background will help you a lot. For me, the test seemed nearly impossible and would've taken a lot of time to prepare, since i have a background in languages and American Studies and I've only taken 7 literature courses, most of which were graduate course on specific topics. For the Subject Test, you'll need to know a lot of basics, not many details. You need to know the basic plots and characters of a wide variety of books (mostly classics), and you need to have some experience with the main poets as well. So, if you have an undergrad degree in Eng Lit, you will probably have done most of this in your courses, so you can take out your notes from previous courses and review them as a preparation for the Test. For standardized testing material, you can search this section of the forum, since we've been posting a lot of information about that here. GOOD LUCK!!!
