NAD 314 blaring volume problem; poor 906 sound

I have two NAD amps with problems. The first is a NAD 314 integrated that, until recently, worked and sounded great. Now, when I turn it on the sound blasts at top volume regardless of where the volume nob is turned. There are also static noises as this happens, that sound like some sort of short.

Prior to the complete loss of volume control, there were no gritty sounds as the volume knob was adjusted, so it's not an obvious case of a dirty/worn pot. I can't connect the 314 to any speakers to investigate further, as it may well blow them, but I think the sound was blaring out of both speakers.

The second amp is a NAD 906 multichannel power amplifier (3 separate amps into 3x2 ch @ 30Wpc, bridgeable to 3 ch @ 90Wpc). The sound I'm getting from it is mediocre. It's not terrible, but compared to the clearly detailed and dynamic sound I get from my NAD 314 integrated (35Wpc) alone or run through my NAD 216 power amp (125Wpc) the 906 sounds flat, rather dull and somewhat muddy. The sound quality is the same whether I use the individual channels at 30Wpc or bridge the channel pairs to 90Wpc, whether the "soft clipping" function is on or off. The sound is no better, and maybe worse, than my old Onkyo AV receiver (mid-1980s vintage). It is otherwise functional.

I bought the 906 used, so I don't know if the sound quality deteriorated over time or if it started out this bad. Is this simply mediocre model? Is there some way I can determine what, if anything, is wrong with it? Are there specific parts likely to cause deteriorated (but not horrible) sound quality after a considerable period of time? If so, how can I go about fixing/replacing them?

Thanks for any insight and guidance.