Its the string gauge combined with the pickup magnets and the winding characteristics that make the differences I appreciate and listen for.
Wow, I am just the opposite. I always play a guitar unplugged before I buy it... so I can hear the wood and the resonance. Though sometimes, it is the beauty of the guitar that gets me, sometime one will seem to call from the wall, like "love at first sight"... if there are several I will still get the one that sound's best unplugged.
J