Hi Eleveniseven,

The ZSP1 preamp kit has a lot of gain making it ideally suited to drive tube gear that needs 2 ~ 5 volts on the inputs.  With regular amps that only need 1 volt, the ZSP1 will be too hot.  It can be modified to use only the two final tubes as a near unit buffer which solves this problem with normal amps.

The CSP2 has an adjustable output level that allows it to be dialed in to any amplifier.  As a line level preamp, it is more transparent than the ZSP-1 with better control over dynamics.  

Even for non-headphone lovers, you may find that having the headphone jack is nice when you hear something in a recording that doesn't sound right and are trying to figure out if it's the recording, amp, or speakers.  

Steve