Why is the Audio Research SP8 so highly regarded?

The SP series, starting with the SP6, was a high-water mark for William Zane Johnson and ARC back in the days of yore! I like that series more than what followed.

I had an SP-8 and then an SP-10 before getting an SP-11. I sold the SP-11 to a friend who still uses it and it still sounds great.

These are all fine preamplifiers. They come from the era when LP was the dominant source, so they all have excellent phono stages. In those days, the integrated phono stage got more attention than the line stage; the consequence was that these preamps simply don’t have the most neutral line stages. They can sound quite pleasant with digital sources, but they’re really not especially neutral.
As good as these units are - and the SP-11 is especially good - they can’t compete with today’s Ref series. It’s not even close.

Thanks for the comments and feedback

Sounds like I need to focus on the Ref series as a replacement. Right now the Ref 1 used can be found for around 3K while the Ref 3 is around 5K. 
The LS26 seems to be a scaled down version of the Ref 3 and is in the $2,600 range. Is there a significant difference between a Ref 1 and Ref 3? Also, keep in mind the JBL 100 Classics are great rock speakers, which is the style of music my system is geared towards; however they are not the typical high resolution type speaker many audiophiles use in their systems.     

cleeds"I had an SP-8 and then an SP-10 before getting an SP-11. I sold the SP-11 to a friend who still uses it and it still sounds great."

That is very funny hear is the thing about user "cleeds" every time any one asks for information about a Music Reproduction System component it happens that "cleeds" either owns or has owned it but that is impossible  because he would have to own thousands of  Music Reproduction System components for it to be true and actually "cleeds" owns no audio equipment at all!