For me, the earliest and cheapest was adding a dedicated line (took one afternoon, less than $100). The most recent, and maybe best, was Krissy's boxes from former PPT: game changing. In between, early-1960 6085 and 7062 tubes (in place of 12ATx and AUxs) for the line stage of a LTA Zotl-10, which itself was a total ear opener. I've also been happily surprised with the SR HFT/FEQ treatment and IsoAcoustic Gaias. All in all, I've been amazed how much tweaks really do matter... I used to shake my head at my brother's Cryo treatments, but now I have an open mind: if I hear it, I go with it. I listen to single-driver speakers with Fostex F166e drivers (4.5") and it sounds like a concert hall.
