Your single most significant purchase mistake?

Thus far it would be a miniDSP unit. I told them what I had, what I was trying to do and they recommended a product which after purchased was told by them it wouldn't work in my system.

Anything class D that I’ve ever taken a flyer on.

Any digital processor or digital crossover.

I’ve taken each of them apart (multiples of each) and rebuilt them as best as possible, still --no joy.

PTSD. I now have nightmares about self powered speakers with class D amps and digital crossovers. Coming after me with axes and the like. Every time we go to a concert, these days, it's all we'll ever get. Live sound (for the most part) has gone totally to hell.

One word. Proceed. I made a massive investment in the Proceed line given the reputation of Mark Levinson / Lexicon / Madrigal. They were new to home theater, and had some really good design ideas centering on bus based architectures for upgradability. The AVP preppro had an available  faroudja line doubler that looked pretty good at the time when it worked, but the sound was a bit sterile and was the best of the components and had to be repaired twice. The transport was in and out of the shop a dozen times until the warranty period expired, that thing never did work right. The amps were beasts but were singularly unimpressive. I was pretty unhappy with the entire setup until I replaced the AMP 3 with Krell mono-blocks, a world of difference. The company just walked away from the product line. 

@zavato sweet Avatar

Rogue M150 monoblocks. I had both cryoed KT 6550 and EL 34 tubes, Harmonic Resonance boards + Stillpoints and Risers high quality PCs, DHL Silversonic XLRs, HQ PLC, 20amp dedicated line. No matter what I tried they sounded rolled off. I was told by the company I bought the tubes from the problem was their choice of transformers

Schiit JOTUNHEIM R ( the only alternative other than the Rall interface box or the Raal  HSA1b which I already have on another system ) This piece of. Schiit malfunctioned as soon as i attempted to power it on .I attempted to talk to customer service or technical assistance  which is impossible because this schiity  company does not have any. You have to use emails that they do not answer .I. had to initiate a PayPal complaint to get my money back and send it back .This company has miserable quality control and it is obvious they do not check equipment before shipping .Normally I would take a chance and ask for a replacement item in exchange but I would never dare deal with this company ever again!!