What sort of sound signature are you looking for?
Actually, all amps sound great with the LS50 and while a better amp does make it sound a little bit better, I found my sweet spot to be the Schiit Ragnarok and anything after that is really minimal. A very good amp within your budget is the Cambridge CXA80, it's not as clean and revealing as the Schiit, but it comes with a decent DAC and is quite satisfying. I personally do not find the Musical Fidelity M5si a good match as it becomes too sibilant/brillant/harsh for me, but you may have higher tolerance.
Also, save some money for a good DAC, I find it affects the sound much more than the amp, especially for the LS50.
Actually, all amps sound great with the LS50 and while a better amp does make it sound a little bit better, I found my sweet spot to be the Schiit Ragnarok and anything after that is really minimal. A very good amp within your budget is the Cambridge CXA80, it's not as clean and revealing as the Schiit, but it comes with a decent DAC and is quite satisfying. I personally do not find the Musical Fidelity M5si a good match as it becomes too sibilant/brillant/harsh for me, but you may have higher tolerance.
Also, save some money for a good DAC, I find it affects the sound much more than the amp, especially for the LS50.