Looking for Better Jitter Reduction in a DAC

It is a bit hard to figure out from this end, of course. I still think you may have a gain mismatch (2.8V into a power amp with a 1.0V sensitivity). It does depend a bit on the design of the amplifier's gain control. Does it come after an input stage? If so, that input stage could still be driven into clipping. A cheap experiment would be to get some inline attenuators from Parts Express: https://www.parts-express.com/harrison-labs-12-db-rca-line-level-audio-attenuator-pair--266-244
Short of using a scope to inspect the signal, you could also listen to a cd with different levels of a test signal, and listen if and when the sound starts to distort.
Alternatively, what you hear with the digital source may in fact be perfectly fine, but unusual for your ears. You hear a difference with your analogue sources, but why would they be superior? Go to a live concert of a symphony orchestra and you will at times hear the pretty harsh sound that some would call 'digital', but that is just the real thing. Do not forget that FM radio is seriously challenged sonically, with high levels of distortion, reduced high frequency response, and limited channel separation. The same is true for vinyl. Both may sound nice and warm, but it is not quite the real thing.

@willemj , thanks. Attenuators may be the easiest way to test this situation. And good point about analogue.
I have season tix to the Philadelphia Orchestra and the brass section can definitely sound harsh at times, plus that is the nature of the trombone or Wagner tuba in some symphonies. But violin in a well damped concert hall should not present harshness, well maybe during a solo.

Steve N, DC offset was checked when I had new dedicated lines installed, not long ago.

Forgot to mention that my SS Sunfire amp with high gain also presented this problem after I received this DAC. BTW, my Chord DAC sounded very smooth.

Well, maybe it is your DAC. A cheap test on that score would be to buy a Chromecast Audio and use its inbuilt DAC for analogue output. It will give you a convenient streaming device in the bargain. The inbuilt DAC is by no means harsh. It has two analogue output levels, one is called high dynamic range. That name is a bit misleading because it is just a higher (2.0V) output level. I would avoid it in your case. The normal one is about 1.2V if I remember well. Once you are done with the testing, you can use the Chromecast's digital (optical) output into a better DAC.

I think the attenuators may be a cheap way to test the analogue output of the DAC. I would really like to find a DAC that I could audition for a trial period, and in the meantime save for a new unit.
As I stated earlier, the Cable Company's fee is too high to borrow a DAC from them.

I think I was able to reload software on my Mac so that I can play from the hard drive. The music players seem to working with no hiccups, so I'll test some downloads.