Until recently I owned 890Cs and I consider them the best "small" box speaker ever made.
The key is the superb 406 woofer which uses a 3" voicecoil, the motor from the 414 12" prosound speaker and a dual profile cone. The cone is straight sided near the center then becomes concave towards it's outer edge. Altec engineers spent a great deal of time on this driver and it gives the 890C the realistic midrange of the large, VOT based Altecs.
At 2000hz a horn tweeter comes in, the "mystery horn" which was probably sourced from Heppner. The horn is very clean with no squawk or screech though not as extended on the very top as some.
The 890C is characterized by excellent dynamics for it's size and a smooth robust tone with excellent clarity. Bass is powerful, clean and fairly well extended. I used them with a Fisher 500B tube receiver, a Jolida 102B tube amp and a Teac Tri-Path amp. All worked very well, the speaker needs very little power.
I sold mine because I moved from a large house to a high rise condo and made a clean break away from Altecs to ESLs (I had four sets of Altecs, the 890Cs the smallest, and one set of large JBLs, all gone now). My 890Cs fetched $386 on ebay.