How important is the pre-amp?

Simply stated, a quality pre-amp is critically important in a hi-fi system.  Audition some well regarded preamps in your system before making the leap. 
BOL

The preamplifier is at the heart of any great hi-fi rig.  A bit of history may be in order here.  In the 60's - the 80's nearly all preamps were full-featured. They included a phono stage, a line stage and a power supply all in one box. There are real advantages to this approach. The installation is simpler, cable requirements are simpler - it has a concentrated purpose.
All preamps of that era had single-ended circuit topology. That was the standard and had the very significant advantage of simplicity of the signal path. More recent marketing efforts promote balanced connectivity. Balanced topology imposes twice the circuitry into the signal path.  Abandoning single-ended is not an advancement.
Goals or attributes of a great preamp:
1) Straight line with gain without contributing to the sound thus preserving micro and macro details, etc.
2) Lo distortion
3) No noise (good signal to noise)
4) Two outputs
5) Adjustable gain and cartridge loading in the phono stage
6) Multiple stages of power supply filtering/regulation and a processor for control
7) Multiple line stage inputs
8) Line stage input with unity gain pass thru for Home Theater
9) Simple remote control
10) Able to drive long interconnects
This is not easy to accomplish by anyone's measure.  The market is full of preamps that fail pretty badly at either one or more of these goals.  After all, there are no standards and specifications are unlikely to give you any scene or what the preamp will sound like. Auditioning several will be best solution for choosing.
All of you in the "balanced only amplifier world" will have to seek out a "balanced" preamp.