1 piece of audio gear I really regret selling is..

Original Keith Monks RCM. Thought it was too tweaky at the time. We were moving around then and every time we did, something would go amiss. Wish I could set the Wayback Machine to the early '90s

Proac Studio 250 speakers. Best I ever had on my modest budget. But I got bit by the bug and tried to improve. Big mistake

My Mirage M760's What a dope I was.
I sold them and bought Defenitives BP10's.

two pieces come to mind--first is my b&k avr-202, which had fantastic build quality, sounded great and had the best learning remote i've ever used. i "upgraded" to a more heavily featured denon, which sounds like shit in comparison.
the second is a cambridge d-500se cdp, which was an essentially flawless player i sold on impulse. i've not been able to replicate it at anywhere near the price.

McCormack DNA-1 W/SMc upgrades.

Wonderfully sweet amp. It matched well with anything I threw at it. Found myself with no choice and had to let it go.

Damn.