I was in the same position as you a while back and quite glad I chose the nfb28. It's now my wife's amp. But I still use it. I now have a master 11. And I do believe balanced is worth it. Each to their own. But other than the obvious Bump on power or volume maybe around 6db gain you might get by using balanced over SE... The sound stage is a bit better imho. And it's quite cheap to convert cables to balanced as long as they are four wire, which most are anyhow.
Recent converted my hd800's to 4pin xlr and boy am I glad. I have never heard them sound soooo good. I almost sold them as SE was ok but not ideal. And that's using it on either the nfb28 or m11. The m11 has plenty of power either way, but balanced just sounds better. Take my opinion with a grain of salt. But most local headfi guys I hang with all prefer xlr over SE. I think there is One guy who likes SE in our local group of audio meets.
If you choose the nfb28, you still can listen with SE and in most situations will have enough power too. But if your hd800s is balanced, then get a balanced amp.
Btw, I listened to the hd800S last night,
enjoy.
Also the abyss, but I'm not thrilled about the abyss.
I own the hd800 and editionX, my wife had the pm3