If you're on a budget, and have to get things piece by piece while looking to rise through the ranks an amp/dac might give it a little more sparkle and also when you upgrade cans you'll already have one to use instead of, say, dropping 300 on a pair of HD 650s and then another one or two on a dac, amp, or a combo. That said an amp isn't going to make a night and day difference unless you're on a pretty weak source so it sort of comes down to what things you want to do, how fast you want to do them and how much cash you ave to throw around.
The short answer is no. The slightly longer answer is they do benefit from one. I have the M50x and I use them with a Fiio E18 as a DAC/amp.With the amp, less bass bleed, tighter low end, crisper highs and more authoritative sound. So o an amp is not needed but yes they are better if you get one that's at least relatively neutral.