Your greatest high end audio purchase!!!

  It was a really nice pair of original Quad 57's that came with a pair of Futterman H3aa's that I picked up in the mid-eighties that I'll count as one.  It was always a love/hate relationship; I always wanted more output out of them and I'd go back to a box speaker but would always go back to the Quads.  To me, within their range, they are amazing in that this fifties technology gets you 90%-95%  of what is available today.  With a modern preamp and source, they shine. I remember listening to them at the local dealer I purchased them from with a Spectral DMC 10 preamp and a fully loaded LP12 back then and it was stunning, even when compared to Modern ESL 63,s and a pair of Entec SW-2's.  I still own the Quads but they reside in my basement with one of them out of commission.  The Futterman's I got rid of long ago as they were problematic.

  Either that or my LP12 which has been with me in constant use since 1986 that drove my view of analog superiority.

  Or my Esoteric K-OX3D / F-05.  The K-OX3D changed my worldview on digital music but with the F-O5 sometimes startles the sh@t out of me.

+1 @rick_n room acoustic treatment -- Without a doubt my best purchase was the MEN 220 Room Correction system. My listening space could not be treated with acoustic panels as it already had wall-to-wall bookcases. The books provide a basic treatment, but surprisingly not anywhere near the revealing benefit of the MEN 220 automatic correction. As a bonus, it also provides active crossover management for my two REL S/510 subs and can shift the sound field with a touch of the remote to change sweet spots in the room.

What "ruined" me was the Audio Research SP 6. Bought it in 1978. Loved it. 

That would be my MSB S-200 power amp; it's my one component I've not thought about upgrading, and I've had it for over 10 years. 

Magnepan 20.1s, purchased like new for $5,500, new would have been $12,500. The imaging is almost always breath taking.