Jolida 1501RC sound distorted after warm up

I notice the unit is running a bit warmer then what I feel is normal when distortion set in with both speakers. Over the weekend I've place the 1501RC in the cabinet by itself without stacking to give it more air to breath/cool. I notice I could run the unit without distortion for a good 2 hours. But then all the sudden I hear distortion again and unit is getting noticeably warmer. This happen when I change the songs I'm playing from Jazz to Rock. As soon as I change the CD back to Jazz then the distortion starting to go away and 1501RC cools down as well.

1) Is this normal? (High & Bass distorted when unit get warmer then normal)
2) Is this normal? (Music preference of the tube hybrid amp between Jazz & Rock)
3) Is this normal? My volume nob has always go up to 1/3 way up with remote and if I want louder than that I'll have to physically turn the nob. I always thought this was a safety feature but I'm starting to having doubt now I'm using the unit a lot more in the last month. Trying to get back to the hobby with 1501RC as my primary audiophile work horse while shopping for better speakers.

Thanks.