Last week Alex Clark wrote of the "Lost Art of Editing" in the Guardian. Presses have been cutting back for some time now. "Many speak of the trimming of budgets," notes Clark, "the increasingly regimented nature of book production and of the pressure on their time, which means they have to undertake detailed and labour-intensive editing work in the margins of their daily schedule rather than at its centre." A freelancer Clark consulted told him: "'big companies used to have whole copy-editing and proof-reading departments. Now you'll get one publisher and one editor running a whole imprint.'"
Clark's mostly talking about literary fiction here. But the cutting back on editing--line, copy, content--is something I've heard about repeatedly from historians and editors at university and trade presses.
You can do a thing or two to counter the trend. Have multiple historians, experts in your field, read your work. Getting far more than your two MS reviewers to take a look at your work will be a big plus. And, readers will probably be happy to have you return the favor for them at a later date.
See if you can get a second copy editor to go over your manuscript. I was able to work this out with Harvard Univ. Press for my first book, The Fire Spreads: Holiness and Pentecostalism in the American South. It helped. The two copy editors caught loads of grammatical infelicities, leaps in logic, spelling mistakes, etc. that I didn't have eyes to see. (It's even worth shelling out the money for an extra copy editor, if you can afford the $500 or so.)
And, finally, ask those at the press that you are speaking to if editors do much "editing." How will your editor help you shape your MS? Talk to authors who have worked with that editor in the past to see what goes into the process. Does she have a hands-off approach? Will she help you craft your argument and ask for important revisions?