Explain what is a clean power amplifier?

With regards to power line noise, on a separate front from distortion, that is not the amp but the electricity that runs it, and a truly clean amp probably has a good quality power supply that does what is needed to feed the amp clean power it needs to perform well. If not external power conditioning should help or be an insurance policy at minimum.

True that both noise and distortion makes an amp "unclean".  The amp itself produces both to various degrees aside from the electricity it runs on. 

The horror!   What a mess!

Amps that are not clean, just need an intervention.
Once clean, they can relapse  But so many amps are in denial.  

Nordost QX4. Enough said.

The devialet expert 120 I'm selling is one CLEAN sounding amp! 

...an absurdvation for one that doesn't use or even have a pre-amp....

I can turn my volume to max....and nothing....even with my low-rent IC's.

AB SS amps....nothing pedigree....SS 'pro' Eqs'....

The RTA sez their's a 'noise floor'....but I can't hear it.

*shrug*  Maybe I'm just lucky....or in a local focal locus that allows such.

If so....no, you can't move here....*G*