Well what are you looking for in an amp? Would you just like more power and volume pumped into your 750s, or did you want a fuller sound along with bass boost?
E11 - good balance of bass boost and volume and price.
Fiio e12- mainly for it's power but has a clean bass boost that extends and enhances the subbass. Not a bass heads boost but a clean improvement on the low end
Fiio e07k/e17 (can be used as dac whenever that comes In handy) but has the strongest bass boost of any Fiio I've tried, has treble boost, definitely enough power for your cans.
To save money -e11. You'll be happy enough with this
For volume and clean subbass - e12
For a very stronger, pretty clean, forward bass boost -e07k
Never tried the e18 but I hear it has a lot of power and has the dac part which some people are using with their phones! Seems cool but can't give valid word for it.