Most eye-opening upgrade and what you learned

There are innumerable, outrageously expensive tweaks and cables available to choose from. In my experience, none are the panacea that some would suggest. The first step is to experiment with placement of your speakers, as this simple adjustment can make an incredible impact on the sound. Make sure your speakers are on a solid footing, preferrably with spikes. Energy expended by the speaker drivers is wasted as distortion of the sound. Make sure your electronics are on a stable platform that is not subject to movement or excessive vibration. This doesn't mean that you have to spend thousands of dollars on a rack made of exotic materials and specialized isolation tweaks. As for power conditioning, having an electrician install a dedicated circuit (20 amp, 10 gauge romex with a hospital grade outlet) for your audio equipment for a few hundred dollars is better than any exotic power cord or line conditioner. Clean the contacts on your cables with Kontak or Radio Shack electronic contact cleaner. Use some decent, preferrably shielded interconnects and power cables, but don't go overboard, as an upgrade of your source component is money better spent. After you have sorted out the basic setup and you component preferences, spend your extra money on tweaks.