McIntosh MC2152 power amp technical question


My MC2152 suddenly acting up. When I use the XLR input the right channel makes popping sound when power off. To make the situation worst, the sound coming from this right channel is slightly lower compared to left channel which resulted in uneven sounds: left sound louder than right. IF I use the RCA interconnect the sound is fine and no speakers popping noise when powered off. I can settle by using the RCA interconnect and move on but i want to use the XLR input for better sound quality.  

Could it be the XLR right channel input connection defect? Or the electric board holding the XLR right input is defect?  Any technicians here in the community, your inputs would be greatly appreciated.  

thank you in advance for all your suggestions/help. 
kentrent
I used to own a ton of Mac.  Send it in! If it is a manufactured fault they will repair it for free, regardless of how many owners there have been.  If you have to pay for a repair, it will be done right and with a warranty.  Their rates are no more than your local repair shop. At least that has been my experience.  They are awesome to deal with.

N.
@normansizemore  i spoke with them this morning they told me no warranty since im not the original owner. I’m not sure if this is a defect or not.  So you dont think their repair rate not that more expensive than local McIntosh service? 
If you swapped the cables and the trouble moved to the left channel why do you think the problem is not the cable or the preamp output? Did you swap both the cables at both ends from preamp out to amp in?
As stated above, if you swapped cables and the problem moved from the right to left, it is not the amps input, but a cable or the preamp / source...
No guys.  Even not connected to preamp still making popping sounds to the right channel. I’m bringing to mcintosh service center.  McIntosh don’t recommend sent it in because its so heavy and transit across country could damage the amp.