I wish I had my Cary SLM-100's back. WOW, did they sound good for how much they cost. They'd be a great start to a 2nd system. Also, some day, when I have the space for that 2nd system, I know I'll wish I had my Quad ESL-63's back.
All that said, I've never sold anything that I wish I could have back in my main system.
Jeff