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
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. 
I am really surprised at Macs response to your service question.  For decades, no matter who owned the unit, if it was a manufactured defect, they fixed it.  Period.  Additionally, their service rates were always in line with third party service agencies, and Mac service was top drawer.  Obviously, the end of an era for Mac, and a huge disappointment to those that have supported the brand.  
@normansizemore yes i am very shocked to find out too.  This is an expensive unit and they didn’t offer me any help at all.  They told me warranty is not transferrable and that’s the end.  Even when i asked to sent in they tried to steer me away.  The whole thing i didnt feel like they’re really stand behind their products.  I am very disappointed in the way they handle this situation.  Usually every other company warranty go with the item until end of warranty term regardless of owner.   Anyone else have any input about this??