Not too long ago, a 'friend' of mine sold me his Adcom GFP-750 for the remarkable price of $200. He was being kind, of course, and he sooo much wanted me to pitch the old, crappy, Yamaha preamp that I had at the time. In retrospect, it was probably more important to him than it was to me.
I was entirely unprepared for the demo. I just knew that there was NO WAY that a preamp would make more than a minor improvement in the sound of my modest stereo. Naturally, I couldn't have been more wrong - and I still have that preamp.