I will talk about the RHA - MA750i first-
Design - Beautiful is the word. It is premium and you can't get enough with the looks and feel proud about the choice of your earphones. I have never seen such beautiful earphones until now. Truly classy.
Build - The build quality is superb.
Sound - I won't go technical since all I care about is good overall performance. Thumping bass and crystal clarity are the highlights of these earphones. There is hardly any genre which doesn't sound great on these earphones. You will need to be patient and give at least 20-30 hrs listening (burn-in) to get these at their fullest potential.
Fiio A1 - This Amp is a great purchase and improve the performance of the earphones to a great extent. However, those on tight budget can easily chuck these and just go for RHA earphones. You will save around 2k and wont miss much. However getting these will be an extra advantage.
So overall, 10/10 for RHA MA750i and 9/10 for Fiio A1!! Go for it!