I look back very fondly for equipment that for whatever reason or other I sold off. Equipment that just was fabulous and made music music music.
What are your regrets? Mine:
Conrad Johnson PV11 w/phono pre amp
Magnepan MG 20 speakers
Peace
Bruce in Philly
Music Reference RM-5 MkIV preamp
Reynaud Offrande speakers.
off topic but if I ever sold my TAD cr1 speakers I’d regret it- even if I bought one of the New PS Audio speakers - would be a dream to have the ultimate point source and an amazing line source .
Krell KSA 250
Sony SCD-777ES, my first piece of audiophile gear.
Air Tight ATM-2. This amp was not only extremely musical, but it kicked butt. Dynamics and PRAT were off the charts. Probably worth more now, then when I sold it.
My Adcom GFP-345 with the MM Phono module and GFA-545II Amp. In Canada, the early 90’s were all about Bryston & Classe’ and I couldn’t afford either. Sonic Frontiers (now Anthem…ugh) was just getting on the scene with their hand made Tube jems also out of my reach.
The made in NJ Adcoms were bought on layaway as I was still in College and used the money from my Summer Student gigs to make the payments. I was going to pair them with Mirage M7si’s but I only was able to payoff the Adcoms and went the homebrew route instead. My Dealer was not impressed but it was purely a matter of cost.
Same here - the piece that sold me on SACD.
Is this an emotional regret or one based on the loss of a hardware appliance that is sonically irreplaceable?
I’ve replaced it 
I felt it was a thing of beauty. It had fixed lasers and moved the disc and disc drive across the lasers. It was built like a tank. At the time it was in the top 3 or 4 best SACD players (at least for consumers.)
Manger Sound System MSS active floorstanding studio monitor… 
I put a fully functional Lenco L75 Turntable with a very nice Ortofon tonearm into the trash. Perhaps the single dumbest thing I have ever done. Oh my god how I regret it.
I saved as a teenager and bought a Pioneer SX-850, a pair of JBL 100s.
Oracle turntable from my late father’s system.
Not that I would need it back, but I would have fun trying again my first high-end setup with the beautiful Linx Quasar integrated from New Zealand and Magneplanar SMGa speakers. It did have impact and a certain speed and especially coherence. Not bad for a below 2k setup.
Berning ea-230 amps. regretted it mightily and have since bought another pair. 
Lynn Sondek LP 12 with Lynn Itok tonearm and Accuphase MC cartridge.
Moved to all digital. Few regrets but hard to see album reissues that aren’t
available in digital formats.
Denon DCD-A100. This was an excellent cd/SACD player with a very robust and accurate transport and a very musical DAC, which could be accessed by other sources. I thought I had an excess of great sources and sold it. . . and since then I’ve missed it and wish I hadn’t let it go as I would now use it in place of another source in a headphone only system.
My Bedini 25/25. Wish I had held on to that, but the price to pay for moving up in audio is sometimes painful.
Ehhh…more nostalgia than anything. I doubt I think it would sound good now.