As
@staticV3 says, audible hiss from the speakers is a very common effect with even some pretty expensive gear.
@Ssmokeyy , can you be more specific about what exactly you are hearing?
- Just to be 100% clear, this is hiss coming from your speakers, not directly from the amp's case, yes?
- If so, is this hiss that you can hear only when you walk up near the speakers, or can you also hear it from your normal listening position when no music/sound is playing?
- Do you notice any difference in the volume of the hiss when you have the MCA50 connected with XLR vs RCA?
If the sound is coming from the speakers but you can only hear it when you walk up close to them (like let's say within a foot or two), it's not necessarily idea but it could be normal.
If the sound is audible from your normal listening position, that could be a few things, including what staticV3 has mentioned.
It could be the MCA50 has a high noise floor.
It could be that the Integra has a high noise floor.
It could be that the Integra's gain (which is a different thing than its wattage/power) is too high for the MCA50 to operate without producing that audible hiss.
Finally, it is still possible that it could be a ground loop. I know from experience that hum is not the only sound that can be transmitted over a ground loop - hiss can too. If it is a ground loop, I would expect there to
also be 50 or 60Hz hum (or maybe a 100Hz or 120Hz harmonic) audible from your speakers. However, it could be at such a low level that you can't easily hear it, or it could be that the drivers and crossover networks in your speakers are such that the ground hum is attenuated so much that it's inaudible, while the hiss goes into the tweeters and is at frequencies where human hearing is much more sensitive.
If it is a ground loop, I would also expect that the hiss would be louder with the RCA connection than the XLR, since XLR has built-in noise cancellation. However, this is not 100% determinative.
One thing to try is to TEMPORARILY install a "cheater" plug with no ground pin between one of the amp's power plugs and your AC wall jack. You should NOT run the equipment this way. But in my opinion it's okay to try this just to quickly turn the gear on to see if the hiss is still there - that should just take a few seconds, as you will hear the hiss or you won't. As soon as you find that out, turn everything back off and remove the cheater plug. If the hiss is gone (or much lower in volume) then it's a ground loop. If not, then it's not.
Only other thing I can think of is maybe to try getting some dummy caps for the MCA50's jacks for the two unused channels. Sometimes an amp can make more noise when nothing is connected to one or more of its channel inputs.