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.
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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. 

Lots of cool stuff.
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.