Im going to buy iBasso D2+ Boa, havent received them yet.
Im pretty sure the soundstage for the e7 is better then e5 considering the price for each items.
I wouldn't venture so far as to declare any upgrade decision as "best" but I sure am enjoying my W1000x much more than my m50s.
The problem however is that the m50s sound pretty close to the W1000x without any dedicated DAC nor amp. Eventually, you'll want to get an amp and DAC if you're still not satisfied. It's sort of a chicken-and-egg problem to me. I'd still want to get a DAC/amp anyways if I were you, if only to see how much more I can get out of the m50s.
Back on the topic, has anyone compared the amp stages of the E5 and E7?