Is it possible your turntable is picking up sonic vibration at higher volumes and then causing the distortion? It is also possible the turntable does not put out enough signal to the 2150 causing you to overdrive the amp at higher volumes. In the first case a bigger amp won't help. You may be able to get a phono cartridge with more signal out. Also recognize that a tube amp with significantly more power than the 2150 is going to be BIG $$$$. When using the digital source how much volume adjust do you use. Is it at 2:30 at max volume.