Who needs a Preamp??

Passive pre-amps have definitely made my system sound worse.  Using no pre-amp with my CD playe,  tuner, and even XM Polk radio was worse, and that is far from great anyway. A far as the electronics and physics involved, please inform me.  I am curious.  I can understand the tuners, because a switch bypasses the probably crappy gain control, and I can understand that my electronics could be better than the passive pre-amps I tried, but no pre-amp with the CD player?

One of the best improvements I made to my system was getting rid of a Conrad Johnson preamp after buying a Lampizator big 7 DAC with a volume control. Removing the preamp lifted a hazy veil and allowed crisper dynamics in more detail. It also put two grand in my pocket after selling the preamp. 

Good move. I made a similar move 30 years ago. Never look back. Something might be gaining on you! 

I was forced to temporarily use my Ayre Codex as a preamp when my CJ preamp went up in smoke (my fault, trying to improve the power supply).
The sound was listenable. I have also listened to vastly more expensive and better DACs (DCS) used without preamps. The sound is always lifeless and flat, the opposite direction from my beloved vinyl and true musical enjoyment - no moments of getting lost in the music. There are reasons, beyond simple controls that every truly outstanding system I have heard utilize a preamp. I believe only theoreticians and misers skip the preamp.

I have not encountered a passive that can drive a high current amp and cables as well as a top tier preamp with a really good VC. As many have stared most commercial units use really poor quality VS's.  What people do not understand is that while a line level component may have enough voltage to drive an amp to clipping it is NOT the typical situation at listening levels. Most people only listen at 1 to 5 watts RMS which is a greatly attenuated level. If you think about it in terms of signal level being transmitted across the interconnect we are talking very low signal levels here that are far more sensitive to ground currents, common mode noise, RF noise and impedance reflections. A really good preamp can deal with those issues with supreme transparency whereas almost every passive outside of a TVC seems to be very sensitive to RF which give a slight edgieness that I feel people mistake foe transparency. Larger amps that draw significant currents also are difficult for a passive as the ground loop induced currents have no mechanism to control them being that the output impedance of a passive is extremely poor.  Now an amp with a well integrated volume control has some potential though