Musical Fidelity M6si cracks and pops and then nothing. What could it be?

Hi all,

I suspect the outcome of all this will be for me to send it for servicing but I wanted to ask in case anyone might know how bad it is and how expensive the repair might be.

I bought a Musical Fidelity M6si around three years ago. Second hand. From March 2014. Worked well even though it had very sporadic usage until recently. As of the last few weeks I've started using it more often. It's currently hooked into a Raspberry Pi via USB which I use as a roon endpoint for music, and then via single ended rca from a DAC where I connect mainly the TV for movies. It's all driving a couple of Sonus Faber Cremona Auditor bookshelf speakers.

Until today the amp had always worked  very well except for a slight humming barely noticeable when there was no sound and completely unnoticeable when the music was on. Then this afternoon I switched it on before a listening session and after a few second with no music playing I started to hear some really odd sounds. It took me a while to realise they were coming from the speakers (I was at the other end of the room and at first I thought something heavy upstairs had fallen on the floor). After that is started to sound like quite loud pops and cracks. 

I immediately switched the amp off. Then on again, and the noises kept being there. Regardless of what input was selected and regardless of the volume knob position the pops and cracks were there (even with the volume at 0). The music was also there but barely audible.

Here's an audio of the problem (not a great phone recording): https://1drv.ms/u/s!AtGhQi5uGwoBhtFVeYE6zaDCO_FK_A?e=dcNo2A

And here's a video of the same issue (again, not a great phone recording): https://1drv.ms/v/s!AtGhQi5uGwoBhtFWPLMA8CzwtzEQGg?e=URskKd

I then started trying to figure out where the fault might be. Here's all I tried so far:

• Changing the power cable had no effect
• Plugging it into a different socket on a different room didn't have any effect
• Tried to swap the usb source. No effect 
• Tried a different dac for the rca. No effect 
• As a matter of fact I tried to switch on the amp with nothing but the speakers hooked up and the problem persisted
• The amp has a line out. I plugged that into a headphone amp. The music was there but heavily distorted. With lots of "static" in it.
• Interestingly enough, the line out test was done a few hours after the initial issue and now the pops and cracks are not there on the speakers. But nothing is sent to them either. No sound. 
• Also while trying out the line out, no matter what input I selected on the amp, only the usb input was being sent to the line out
• One last observation: when testing and changing the input source via the front panel buttons, one could hear a "thock!" sound over the "click". It's slightly audible in the audio attached to this post.

So it's now all unplugged as I want to avoid any further damage. I don't think the speakers have been damaged but tomorrow I'll maybe try them out on an old Marantz PM amp I have in the attic. 

Any ideas on what could this be? Might open the chassis anyway and have a look tomorrow since it's out of warranty. Found a distributor in the Czech Republic who can probably help me with the repair but any insight you can get me would be much appreciated.