The reason you don't hear the hiss on snares, hihats etc. is because these sounds have stuff going on in the same frquency range as the hiss, this effect is called masking.

My 60 has always hissed. Might be that it's a problem with mine too, but i like it. If you want hi-fi audio quality a 12bit sampler isnt the best choice

Maybe you bought new monitors or did something else to improve your listening situation ? Chances are the hiss has been there all along, you just didnt notice it until now.
Sample with as strong as signal as possible, gain it until it the sampler distorts and then pull it back. If you sample a weak signal and amplify in the mpc you bring up a lot more hiss. If your moving samples from a computer check the levels in some sort of audio eiditing program, the loudest point should be just below 0dBfs.
Some tips, which i'm sure you already knew, hope you sort it out.