Loud popping noise with Classe Audio SIX pre-amp in phono MC mode

Dirty volume control?

Sounds like a cap is going bad. 

Could be resistor, cap, or loose solder joint.  Nice preamp, do you have someone who can work on it nearby?  Most likely an easy fix.

I agree with pops,  I had a similar problem with an older McIntosh pre amp.  The problem seemed to go away after it got warmed up and it turned out to be a loose solder joint.

Hello,
sorry for my late answer and have many thanks for your help.I opened it last week and noticed that 1of 4 Capacitors (4,7 uF /63V ) between GND and TT GND were blown.Yesterday I replaced all 4 caps and all Caps 4700uf (blue Philips Type) with new ones(Roederstein) . Today I will test it out.You´ll here from me soon.

Forgive my inexperience but how did you know they were defective? 

the 4,7uF were exploded , don´t ask me why .But there were no left over parts in the unit, I think this happened to the owner before.The replacement of the other Electrolytic caps are more a general overhaul (more than 25 years in use)than a defect. I have not much trust in the blue philips caps so I replaced the most by roederstein capacitors.I´m working as technician for Lufthansa and repair cabin electronic, former Ißve been working for Kenwood and Akai.But with I am very unexperienced with problems like the popping noise only in MC mode.Never had  a problem like this before with all the amps I repaired in all the time. Really strange ....

Thanks! Glad it's fixed.

Impressive self reliance!  Good to hear up and running.