wrote in message
ups.com

I haven't been playing nice with it recently. I have run a microphone
and bass guitar through it. However, the amp had experienced the
problem I described above before this.

It's broken!

So my question is this: Is there any fundamental reason why running a
bass guitar or mic through this amp would hurt it?

No, no matter what someone may have told you or your guilt-wracked mind is
telling you.

One of the *loverly* things about electronic gear is that it can break for
no fault of your own. It can even break while it is not being used.

Basically, all the angst in the world isn't going to bring it back. You need
to fix it or throw it away or sell it as broken, or fix it yourself. Given
the questions you are asking, you lack the skills to effectively fix it. So
either find a qualified repair person or dispose of it some other way.

Also, does anyone have any idea what might be wrong with my amp.

Whatever it is, it is affecting both channels, per what you are saying.
Since fuses aren't blowing, the problem might be in the low-level input
circuitry rather than the outputs.

I see that a NAD 216 is going (or to be exact not going - zero bids) for
about $500 for eBay, so there is a possibility that your NAD 214 might be
worth a fairly expensive repair.