Should I eliminate my preamp?

The advice given is good.   If your inputs are limited to one and that is the digital input with variable output, you have nothing to lose by using it instead of a preamp, unless the digital volume control were noisy for some reason.

I have one preamp, original cost $5000 (which would be around $10K today) sitting in a third system, just because it has a very good dual volume control with discrete high quality resistors and a switch. The fancy dual phono stages etc. remain unused.

Wonder if someone has done an A/B between something like that and a CD player digital volume control.......

Never get rid of your pre amp , You want the best ? Go with BAT Balanced Audio Tech.

What Ralph mentioned about coupling caps above is not relevant these days, as nearly all solid state sources have been direct-coupled for years,

This statement is false, on account of the fact that a lot of solid state equipment does use coupling caps. Some does not- and then your chances of successful passive operation improve.

It's always been said by audio designers as a gauge of how good a preamp is since the mid 1900's, when separates (preamp & poweramp) entered into audio, that saying is:
 
"The best preamp should sound like a piece of wire"
 
ie: a like a direct source to amp connection.  

Just because one might want it to seem that way does not mean that is what it should be. Interconnect control is why so many people on this thread have had success with active line sections.

The bottom line is care and audition is imperative if you want the best out of the investment dollar in your equipment!

George, Ralph, thank you both for taking the time to explain the math. I had to read them twice to understand the details but it does make a lot of sense. It also explains why some folks report that using different interconnects makes a big difference in sound quality while others don't hear a difference.

It also explains why some folks report that using different interconnects makes a big difference in sound quality while others don’t hear a difference.

You welcome, and correct, always use quality low capacitance interconnects, with passives and you’ll get the closest to the proverbial  "The best preamp should sound like a piece of wire" sound.

Cheers George