I know these have been reviewed extensively in the past but I thought I would share my experience for those that care.
I always wanted a pair of these after reading about them in the mid 80's. When I went from Maggies to a book shelf speaker from 1990-1993, I opted for a pair of Spica TC50's. I had moved from a big apartment in upstate NY to a small apartment in Manhattan and there was no way the Maggies could be used. I never regretted my time with the Spica's and they were generally regarded as an exceptional speaker. However, what I really wanted was a pair of Celestion Sl6s'.
In 2016, I finally picked up a pair and immediately out what others liked about them. They sound surprising deep for a small speaker and the highs were good. Don't buy into the bad rap that metal dome tweeters get. When executed correctly they can sound great. Also, these are a fairly well balanced speaker, not the ultimate in see through resolution but good, never the less. And, in my opinion, they walk all over Klipsch Heresy's. Which I played with and struggled to get them to sound right. In the end I was able to get the Heresy to sound fine but ultimately they could not do enough right, so I sold them. But, here is a speaker (Celestion) right out of the box that sounded pretty darn good.
What they do struggle to do is play loud and who doesn't like loud? When pushed hard they tend towards distortion that comes across as compression. The midrange get a bit muddled, too. Well, being the tinker that I am I decided to mod them a bit. Nothing too radical. The radical mods were all used up on the Heresy anyway and here is a speaker that already was most of the way there.
I order some new silver plated copper and teflon hook up wire and new capacitors to replace the old ones. Honestly, I was wasn't expecting them to improve in their ability to handle louder but, rather, thought that after 30 years it might be a good idea to replace the old caps that I'm sure had drifted out of spec. While I was in there I might as well upgrade the wire, too.
After a couple of hours the upgrade was a done and I hooked them up. Wow, did they sound awful! Honestly, I thought they might sound a bit off at first until the caps had a chance to burn in but they sounded really awful.
It didn't take long for the caps to stabilize and in a short time, within 2 hours of continuous play, the speakers began to improve. Over the next few weeks they continued to progressively improve in all aspect of reproduction and when the family was away I decided to turn up the wick. I totally expected them to poop out like they had before. After all, this is why Celestion made an aluminum cabinet for them and then sold them for more than double the price. What I got was a speaker that no longer pooped out but played very well into the mid 90's. Who would have thought that the caps and wiring would have greatly improved an already good speaker in this area?
Anyway, over the next month I listen to them coupled to 2 MK 150's subs and enjoyed every minute with them. However, I can tell you that they can be run to great satisfaction without subs but by adding sub that last bit of low end power is there.
I'm most likely going to keep them for the long haul and have been pondering the idea of putting an exotic wood veneer on them. Not that the teak doesn't look good but a burled walnut, like what I put on the Thorens TD 150, would look great.
Finally, a few last words. To get the best out of these speaker they need amps and watts. I ran them without issue with 120 watts of ultralinear tube power from my Rogue M120's. I can't say enough about these amps. They sound great and you get your choice of 60 watts triode or 120 watts ultralinear. The 120 watts have been enough to drive the Celestions and my Maggies, while putting out glorious sound. The 60 watt triode mode is way more than enough for my Focal Micro Utopia's, which are a very easy load.
Bottom line, my most highly regarded recommendation on picking up a pair of these speakers. They can be had for a very reasonable dollar and will play splendid, as long as you have the power to drive them. 30 years later is not too late, is what I say!