Are Pre-Amps necessary?

Cerrot phrased it nicely "It gave flesh to the musical bones" Well said...

As mentioned, this has had a lot of coverage here.  Assuming you are talking about a line stage and not a preamp with phono, then there are several functions.

• source switching
• volume control
• voltage gain
• impedance control
• interconnect control

Assuming your source has sufficient voltage, then passive units, or DACs with onboard volume control, can get the first three done.  Impedances can be mostly matched through your source/drive unit and amplifier selections, and short connecting cables can help with the interconnect control issue.  However, even with short cables and more than sufficient voltage, many here find that an active or at least buffered stage improves body, tone, and drive so, at least in some systems, sufficient voltage alone is not cutting it.  I have tried multiple passive units, and even a DAC with onboard VC and a 4V output, but always come back to a buffered unit sounding clearly better to me.  Borrow a passive unit and make your own decision about what sounds best to you, in your system.

There is a Youtube video from PS Audio out there. John M. seems to espouse using a preamp than not.

As I am using streaming, I really need to try using my streaming app (Roon) vs my preamp.
Though I expect the result to be much like 'Salt to taste'.
B

IME this always comes down to cables. I used to know this guy named Robert Fulton who, more than anyone else in the world, founded the high end audio cable industry. He had his special RCA cable that sounded better than the average Radio Shack cable at the time. It was a bit of a revelation...

Here we are 43 years later and cables are still a thing. When you run passive controls you are more subject to the coloration of cables. But there was a tech that was developed to eliminate cable colorations. That is the balanced line system, which has a set of standards that have to be met.

Most manufacturers in high end audio ignore those standards. So we have audible differences in balanced cables too- sparking the question of whether balanced line cables are even better at all. If the standards were being met this would not be a question! But to meet them, you'll need an active preamp to drive them. You can't do it properly with any passive volume device made.  

"...Never heard a pre that "Improved" the SQ compared to "No Pre"..."

Have you tried a Conrad Johnson GAT?