Are Pre-Amps necessary?

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.

Many believe having no active  preamp or linestage in the signal path is the “purest” appproach to home audio.  However, the electrical parameters of the cables themselves can result in colorations.  The balanced line standard Ralph often discusses provides  benefits in reduction of common-mode noise and in cable drive abilities (due to noise cancellation) but must meet certain requirements, which would be difficult to meet when using components from different manufacturers, because of differences in impedance and because not all equipment that has balanced connectors is actually a differential balanced design.  Some additional information for anyone who is curious.

A balanced transmission line consists of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits.
  

Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by transformer coupling or by merely balancing the impedance in each conductor.

Compared to unbalanced lines, balanced lines reduce the amount of noise per distance, allowing a longer cable run to be practical. This is because electromagnetic interference will affect both signals the same way. Similarities between the two signals are automatically removed at the end of the transmission path when one signal is subtracted from the other.

Lines carrying symmetric signals (those with equal amplitudes but opposite polarities on each leg) are often incorrectly referred to as "balanced", but this is actually differential signaling. Balanced lines and differential signaling are often used together, but they are not the same thing. Differential signaling does not make a line balanced, nor does noise rejection in balanced cables require differential signaling.

Now if you have a DAC with multiple inputs and a volume control on it, designed to drive an amp directly, the simple fact is you have a line stage with a DAC built in. But like it always works when you integrate things like that, if you want to improve the DAC, you'll be changing the line stage too.

I have owned a DAC with a passive VC, but do not remember seeing one with an active stage.  I am surprised manufacturers of those DACs with VC don’t provide a unity gain buffered output.  Could be a second pair of output jacks, or maybe switched, passive/buffered.  The passive crowd wouldn’t see the need, but some might be surprised with the outcome of a direct comparison.

George, it was the Metrum Acoustics Adagio, which  changes the volume by controlling/changing the reference voltage to adjust the output voltage of the DAC.

George, not sure where your list of "all those preamps" came from but of those, the Khozmo (i.e., Hattor) is the only one I have owned.  I do own McCormack's TLC-1 passive/buffered stage that Steve has upgraded to a much higher level that he calls  "very close to my VRE-1."  When I owned that Metrum Adagio DAC with internal VC, I modified the TLC by taking the Shallco VC switch out of the circuit (replaced it by soldering two AN Tantalum silver resistors to the board) thus turning it into a unity gain buffer w/o VC.  I did that because, to me, the Metrum DAC sounded noticeably better with the active buffer in the chain, even though the DAC output 4V.
I have since moved to Mojo Audio's EVO B4B 21 DAC, which does not have a VC so I added the passive Khozmo unit, which provides remote VC, balance control, and a nice visual display.  I have tried running my system with just the Khozmo passive unit but I much prefer the sound with the TLC active buffer also in the playback chain.