I saw this question on the Audio Circles forum and found it interesting to post here.

So what was your most dissapointing purchase for your system?

Mine have been the modifications of my Music Hall 25.2 cdp from reference audio mods, my Monitor Audio S1's (should have gone with the PSB B-25's or NHT Classic 2's)
the Electro Harmonix 6CG7's tube's that I recently bought in my tube rolling endeavor (too sibilant), and the Marantz SA8001 SACDP that I sold.