Where did you sink the most cash into?

For a while it was Super DH Cones, I must have had fifty in the room, under components, under speakers, under furniture, on top of speakers and under stacks of those 4" ceramic tiles that were kind of a passing fad about ten years ago, sometimes crowned with Brilliant Pebbles. The Supers ain’t cheap, either, $150 for a set of three.

+1 sksos1

Second was the kids!

Preamp.... It's the most important component in  my opinion..  My speakers ,amp , and DAC were a little less and were about the same price each.  

Of course speakers are the most important but there are a lot of great ones out there.  Upgrading my preamp made such an improvement that I put off buying new speakers in favor of a new DAC.    I ultimately upgradedecided my speakers but honestly don't think I would have benefitted as much if I didn't upgrade the preamp first .

The most expensive piece I own is my pre-amp.  Most expensive new or used (as I bought it).  My speakers are DIY bought used, but given the cost and relative rarity of the drivers, they would be the most expensive piece to replace if I had to build them from scratch.