What interesting or unusual gear has passed though your system during your audio life?

Just curious...any regrets about gear you moved on from?

For me personally, I wish I still owned my Threshold S-150 series 1.
When you say "unusual" that would be a vintage Fisher audio solid state amp. I don’t remember the model number. Grandparents gave it to me as a gift. (had it sitting in the basement for a long time).

The amp was large and held some big capacitors; coupled with a big ole power transformer.

I was only 16 when I got it from them. And I knew then it was just awful. The headphone out could barely drive any of my headphones at the time to sufficient level. The bass was warm and loose-sounding, and the midrange was recessed with treble frequencies rolled off.

Overall, the worst amp I’ve ever heard. Tested with KEFs, JBLs, and Cerwin Vega made them all sound completely lifeless. (mind you, those were my early days!) lol
Had a battery powered MC demagnetizer. Hooked it up a few times, never seemed to make any difference. Opened it up, pretty impressive looking circuit board, LEDs and everything. One day I thought, well if it's doing anything it would be generating a sweep tone or something. Hook it up to the phono stage and.... nothing. Nada. Not doing a damn thing. Did it ever? Was the whole purpose of the board to light up those LEDs? Dunno. 

Maxell AE-320

No regrets, but I would not recommend it in 2021. I still have it. Box, instructions, small brush, cover/stand, mint condition.

In the similar tone, TDK HD-30 (red type). It did make sound and it did have a LED. It still looks and works as new. I mean, the LED still lights up and chirping sound is still there. I never noticed difference after using it.
An N.E.W. DCA-66 amplifier. It’s powered by four 18 Ah batteries. Didn’t sound too bad. It’s not in my system right now but I still have it. It was making some strange noises and I need to investigate the cause. Hopefully it’s a small issue.
@mkgus  I also had NEW DCA-66! Recently sold mine to a guy who was going to refurbish. I had issues with the charging system, can't recall the exact issue, want to think it was diode issue. Mine was making strange sounds when I had the issue, screeching sound if I recall correctly.
Interesting, that’s the same sound I am hearing! How did you like the the amp? Due to some issues with my preamp, I feel like I never got to drive it properly and never truly heard its full potential. If you ever hear back what the exact issue was, please pass that information along. I would like to get it up and running again one day. 
Just wanted to chime in to say I replaced two of the batteries and the amp is working flawlessly now. My issue turned out to be no big deal. One of the batteries must have gone bad. Figured if someone came across the same problem, they would like to know what worked in my situation. I also heard from the previous owner that the built-in charging system can be hard on some batteries, especially when float charging. They recommended leaving it unplugged once the batteries were charged up. I plan to keep an eye on the voltages and only let it charge when the batteries need it.
There are several pieces I regretted but nothing I want back or miss. Even with several bad moves the overall trend has been steady improvement. I still have a few moves left.

I sorely wish I had my 1996 911 Turbo back. I had to sell it because of financial difficulties. 
Good that only batteries at fault. I just recalled I heard the same screech sound a second time, I then put my CJ MV55 into service as replacement, never did go back to NEW. I bet batteries may have been the only problem for guy I sold it to these many years later.
I never did hear the full potential of NEW, it was in one of my early systems, curious as to how it would sound in my present system.
One common misconception about the NEW is that it was Nelson Pass design, it was in fact only Pass influenced design. I purchased mine in 1996, interesting design, and how I learned about how dirty AC affects sound quality. I actually went to in home audio dealer intending to purchase VonSchweikert speakers, came out with NEW.
For me I would say Nestorovic 5As speakers. I had heard them at a dealers once and thought that they were amazing, and years later had the opportunity to buy a beautiful pair for a reasonable price. They were sent to me by the former owner who was a doctor packed terribly in loose boxes wrapped in old rugs, but miraculously were in perfect condition.
I never got them to sound as amazing as they did at the dealers, but they were very special in some ways, and I am happy to have the memory of them and the time I spent with them. 
Interesting to hear Threshold. I guess for me it would be an old Threshold T200 that just wasn't a good sonic match for ML SL3 electrostatic hybrids (out of the room in twenty minutes).     I wish I still had it today, but back then it was worth too much money too keep it and not use it...