what are your top "audio bargins" in your system history?

To this day I have no idea why the seller priced a 9.5 out of 10 Rotel RHA-10 preamp at $300, but I do know it wasn’t stolen.

I hooked it up, turned it up, then looked it up; there was only one on sale in all the world, in Germany for $1300.

My whole system from the used market. Intergrated amp at 45% retail,  steamer-dac at 40% retail, speakers at 40%. 33 grand system for 15 grand. Plus 10 grand of cabling for 3. Only way to go on a disability pension over time. I was lucky enough to sort out getting my dream speakers when they became available, which was the game changer for me. Was seriously hard to do, but well worth it to me.

My neighbor moved out and threw out a Dynaco ST-70 and PAS. Hooked them up to a pair of large Advents my cousin gifted me. Still going strong!

I search for deals anytime I buy anything, which generally helps me get something twice as good as I can actually afford at that time.  My two favorite finds were completely by accident though.  One was a Dual record player that was in the prop room at our high school.  The director thought it was broken and said it was donated for a past show, but if it worked I could keep it.  It worked perfectly.  The second was a pair of Dahlquist M909 speakers at a garage sale on my block for $150, essentially in perfect condition.  Before I found them, I was stressing about what I was going to get once I finished my basement renovation.  When I picked them up, I figured at least these could take care of us until I figured it out.  They sound so good, I no longer feel any need to upgrade.  It really feels like an endgame speaker for me. 

In my case it would be a tie between a Yamaha CT 1010 I bought second hand for about $30 in the mid eighties, and a Sumo Athena, which I picked up around 1992 for $100. Neither are the best of course, but I still use them every day, and have built the rest of the many system iterations over the years around the phonostage built into the Sumo.

She's put up with all other replacements over the years, until seeming to say "all right, that's enough. It's my turn again".