Why is the Audio Research SP8 so highly regarded?

I have a MRk II SP8 that was modified by Steve Huntley when he was at Great Northern Sound. I don't use the phono section anymore as I have acquired a ARC PH7 so the SP8 is now simply used as a line stage preamp. Am considering replacing with something like a LS26 or Ref 1 { both are in my budget range} Interested from those who might have experience with the SP8 and can offer advice on how much sonically the newer ARC line stage models will improve overall presentation from both analog and digital sources?   
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A little info about the rest of your system helps to get responses that are meaningful. For instance, your SP8 is single ended and the more recent models are balanced. Will you be able to run a balanced system with either of the new preamps?
I have the same question regarding the SP-10 MkII.
I find modern AR gear to be much better sounding. I found the original cold, sterile and perhaps lean.

The modern AR gear looses the lean and cold, but is it better than just neutral?

Listen to Ayre and Luxman for alternative presentations which aren't too euphonic. :)
I had an old AR SP8 back in the day mated to a pair of Cary Audio slm 70 mono blocks driving a pair of custom speaker I built, man what glorious musical sound.

Later I had one of the first AR Ref 1 and it was better in everyday as I recall, transparency, soundstage you name it. Cant comment on the LS 26 but Im partial to the ref series pre. Never heard a cold lean sounding SP 8 or SP 10 pre. to me they where glorious tube sound for their time.

Times change.
Was it twenty-five years ago?  Was it thirty-five years ago when I heard an AR SP8 in a sound system at a long-gone L.A. stereo store?  Anyway, this was during my NAD 3020 days and I totally swooned when I heard the SP8 coupled with its companion power amp. Lean? Uh-uh. Cold? As a summer's day.  I dreamed about that damn thing for years but could never come close to affording it until the model was essentially a museum relic.
I found the original cold, sterile and perhaps lean.
Wow! Never heard this one before!!!! Nothing can be further from the truth! Sterile is 90% of today’s line and phono stages that can not even begin to give the listener any hint of the portrayal of space.

I had much experience with the SP8 with frequent listenings at a dealer. And I owned the SP-10 for 8 years in the 80s to early 90s. They both emphasized the middle 5-6 octaves which clearly brought on a most romantic, addictive, euphonic result. Sadly, this came with much compromise at the frequency extremes.

Both of these units were ALL about phono playback. They were a bit noisy so a constant update of the phono’s first stage tubes was imperative. The RAM ultra-low noise tubes back in the day were mandatory here.

Once the use of other sources such as CD players or tape playback came along, these units quickly showed their weakness to the units coming along in the mid 90s and beyond. And by then the ARC LS1, LS2, LS3 were absolutely horrible .... these were cold and sterile and dimensionally flat. Finally ARC brought the magic back with the LS5 and then continued on with the 25/26, Ref2, etc.  To use the SP8 or SP10 as either a line stage or phono stage today just makes no sense; far too much progress has been gained in the last 35 years.

I ran with the LS5/PH5 pair for another 8 years until I moved onto the BAT 31SE/P10, Aesthetic Callisto/Io, Aria WV5 and now Zanden for phono. This has been an ever increasing magic of 3D portrayal with tonal coherency, clarity and resolution gains at each step.

I did try the Ayre as a dealer demo, and this was way too dimensionally flat for my taste.

Just a little more context on my system. Currently running SP8 into an ARC VT100. My speakers are the JBL Classic 100 reissues. Sources are PS Audio Direct Stream DAC and PS audio transport. For analog I run the Dr Feickert Woodpecker with Soundsmith Zephyr cartridge and ARC PH7 phone stage.   
OP I bet that is a fantastic sounding system :-) a Ref 3 would be awesome IMO
( I am biased w a REF5se )
I agree a ref 3 would be a good candidate to try in ur system.

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!