Explain what is a clean power amplifier?

You will get a slew of answers to the effect that it depends upon measurements. But, that doesn't guarantee an amp will sound good. It's been known forever that tube amps typically have more distortion, but many love the tonality despite them being less clean. 

One man's "clean" amp will be another's "sterile" amp. 

Expect many recommendations of favorites purportedly clean amps. This, as well as any other parameter of sound quality, is on a spectrum - a wide spectrum of performance. 

Just compare a few and go with your favorite. 

You people are fooling yourselves in an effort to not spend money - your higher priority than actually attaining superior sound.- by promoting your belief that at best average gear will magically get you superior sound. You are completely clueless. You’re wasting your life fooling with poor methods that waste time and money. Never seen such arrogance over nonsense. You will NOT ha e anything close to SOTA, no matter how much you boast. ONLY superior systems make superior sound. Tweaking is a loser’s game, for those who have resistance to spending money but want to pretend they are doing the same as those who actually go out and get premium sound.

Simply because you work like a dog to tweak systems, you figure you are making amazing headway, catapulting the sound WAY up there. Not so. You are messing around in a small, yes small, slice of the performance spectrum and are not even touching the upper end. You typically do not qualify your posts, but boast about getting sound that can't be improved, is exemplary, etc. It's ridiculous.  :(

Mahgister, you typically make claims that spending more money without doing your "embeddings" is a waste. That's wrong, completely wrong. You are not making major progress by tweaking out your room. Even a very inexpensive speaker, one costing not much more than what you have now, with a different driver configuration, one that doesn't have to be put on a desk, would transform your sound utterly, completely. You would be in an entirely different place in the spectrum of performance. You could clear out every one of your "embeddings", put in a better speaker, and the sound quality would soar far above what it is now. 

Tweakers think they are moving at light speed toward the upper end, when in fact they are inching along. It's pretty sad. I used to be a Chintziphile, so I know all about pretending you have oh, so close to premium sound on the cheap. 

Go ahead, dismiss my comments as though I have something to sell. I'm not paid to write, and I have put in a lot more of my own money into audio than I ever dreamed I would. The reason? I actually got to handle the gear that showed how wrong I was. If not for that I would still be in the camp of people thinking that you can get the same result on the cheap. 

rixthetrick, I'm disappointed in your characterization of my spending on audio. You are not aware of my personal finances, nor of what you see that is on loan for review vs. what I own. I would appreciate it if you refrained from attempting to judge my personal decisions on gear. Thank you.  :) 

One more comment; for the record (again, as I have said this publicly prior), I have never berated or mocked any low budget system.  Rather, I have consistently encouraged and complimented persons who I felt put together impressive low budget systems and said that I thought they would enjoy them. 

I will never forget that more than 40 years ago I started out with a Radio Shack receiver and two mismatched, naked full range drivers taken from console stereos and hooked up with lamp cord. So, please, let's not have derisive comments about my showing disdain for budget audio. 

I have, over the past 25 years, budgeted literally a line item for audio and invested into my system annually. That is one sure way to end up after many years of working at it, with something better than at first. Putting my time in learning, designing and building a specialized audio room was well worth the effort and cost. It was one of the most rewarding things I have done in the hobby, and the satisfaction of using the room regularly is quite high. I highly recommend to those with the space and inclination to give it a go. 

BTW, there is recurring insistence on this forum to the effect that the room is the most important part, that a serious audiophile needs a treated, dedicated room, etc. Nonsense; I oppose those sweeping comments, partly because I enjoyed audio in a lived in part of the house previously, and because I think they vastly overrate the impact of the room vs. the benefit of advancing the system itself. I think it would be very discouraging to the budget audiophile to see such comments, and I do not support them. Of course, a dedicated room is a bonus, but it is ridiculous to declare that one is not doing high end audio if the room is not treated with specific treatments, etc. Imo, that is a form of dismissive elitism. 

So, anyone who suggests that I'm putting down budget audio has no clue regarding my disposition.   :)