I'm a long time fanboy of MBL.
First time I heard them was at an early CES - the 101D I believe. I heard what sounded like actual live music coming from a room, entered, and saw the unbelievable looking pods-from-mars. Some jazz piece. I couldn't believe how live it sounded. Specifically, I remembered noting that for the first time drum cymbals actually sounded like drum cymbals, like the big resonating metal discs they are, rather than the small "spots of bright sound" coming from most systems. The MBLs to me made most of the regular speakers sound like woofers 'n tweeters in boxes.
So I became besotted. Next I was able to hear the MBLs in Absolute Sound reviewer Michael Gindi's room. He had them in a small acoustically treated room and the demo he gave me was mind blowing. The most realistic sound I'd ever heard.
Next I heard them a few years later at some dealers and again, like nothing I've heard - it seemed to transcend the usual breaking down of things in to "details" to just like hearing things in real life.
Then not much later I lost a heart breaking once in a lifetime deal on the MBL 101Ds on audiogon. I was bidding for them, sent off my big bid in the last 15 seconds but a glitch stopped it from going through, and the other guy got them. Still hurts!
A while later I happened upon another killer MBL deal: A dealer had shipped some MBL 121 stand mounts that got damaged, so he had to sell them off. The grills were toast, and the speakers had a few nicks, but I got them for around $3,000 when they were going for $18,000 new! And they looked from any reasonable distance brand shiny new! That was the best audio deal of my life.
I owned the 121s for about 10 years. I used solid state (Bryston or Harmon Kardon) but mostly tubes - I'm a tube guy and my room wasn't big so didn't need tons of power. I found they were just right with tubes for my taste (CJ Premier 12 monoblocks, though I also loved them with a little Eico HF-81 14W tube amp!)
They produced the most life-like sound I've had in my room. An acoustic guitarist would just "materialize" in the room, 3 dimensionally, and with an effortless level of natural detail and timbral realism. I love speakers that soundstage and image well and disappear, so all my speakers have had that quality (e.g. Audio Physic, Von Schweikert, Quads, Waveform, and many others). Whenever I'd think a conventional speaker was disappearing and imaging well I'd fire up the MBLs and it would remind me: they were in another league.
Eventually I fell in love with some Joseph Audio Perspective speakers and had to sell gear to afford those, and so I finally said goodbye to the MBLs. I could have sold them for more than I paid, but I kept the same price figuring it would be nice for someone else who pined for MBL, but they were always out of reach, to get a chance at them. I put them on sale and on the same day got something like 36 eager offers to buy them immediately. Never seen that much interest in a piece of gear ever on the used market. They went to a very happy young audiophile.
I was ok giving up the MBLs for the Joseph speakers because the Joseph speakers came so close to the timbral realism of the MBLs, and the disappearing imaging act, but added more bass and an overall more "juicy/punchy" sound.
Still, sometimes I wish I still had the MBLs to re-visit their unique qualities.