Your amp is real quality and at 40 watts is no slouch. However, if you turn it up it begins to run out of grunt at some point. You would expect that to be when the volume is all the way over, but it isn't always. It's just like a car really. A small engine runs out of push and starts to sound strained at some point when you are accelerating. It may sound strained at a different speed if you are going up a hill rather than going down. Amplifiers driving speakers will get strained at different at volumes depending on the characteristics of the speaker.
Amps have a particularly difficult job driving the woofers (which produce bass). The woofer is moving a great deal of air in order to produce bass. This puts a strain on the amp and is heard as distortion when the volume is high.
The answer is to run your amp at below the distortion level, or get a more powerful amplifier.
The other thing that distortion can do is damage your speakers. Your speakers, for instance, are capable of dealing with amplifiers more powerful than your current one. However, if you run the amp too loud and distortion happens, the woofer moves in an uncontrolled way which will rapidly cause damage.
Hope this helps. If it doesn't, put your question a different way. It is probably us being thick and not understanding your question.