1970's Audio Research Equipment, worth having?

Having owned a lot of the gear that Audio Research built in the 1970's and early 1980's I can speak from experience....
Pass on the D-100, D-110, D-111, and D-350 solid state amplifiers as they were all early efforts into solid state design from Audio Research...and the sound is disapointing. Later Audio Research solid state amplifiers built in the late 1980's and 1990's were much better, such as the D-130, D-200, D-300 and D-400.

The D-79 and D-150 which were originally developed in the 1970's are classic/collectable Audio Research designs and hold up fairly well when compared to later units. I always felt the D-50, D-51 D-75, D-76, D-90, and D-160 had that "soft tube" sound and lack the high definition of the D-70, D-115 and D-250 series.
I can strongly recommend the D-70, D-115, D-250 and M-100 tube amplifiers, as they were a huge improvement over all previous ARC tube designs. That series of amplifiers, featured new transformer designs which gave them better frequency response at both ends.
In terms of preamps...I have owned a couple of SP-3 and SP-2 preamps and when compared to the SP-6 series or SP-8 series they sound pretty dated. The SP-6 and SP-8 offered a large improvement in definition over the earlier ARC preamps...the SP-3 and SP-2 were very soft sounding in much the same way as a Marantz 7 preamp sounds. The solid state preamps that ARC built during that time, such as the SP-4, SP-5 and SP-7 are "not bad" but the tube units of that era are much more musical.
Of course, the SP-10 preamp is considered by many to be one of the best preamps ever built and the SP-11 was also a great preamp too.
Hope my input helps on this older posting.

I have owned an SP-3A-1 since 1974, and in 2003 I had it upgraded to an SP-3c. I also own an LS-15. Lately I've gotten the bug to upgrade. I am thinking of selling both, adding a few $, and buying an LS-26 (used). Any thoughts ? Would the LS-26 be a big upgrade from either the SP-3c or the LS-15 ?

Why do member's re-open dead threads that've had no action for over three years?

I've never read the thread before...so it's new to me.

Put me in the ARC camp.   I hear the SP-3 and power amps driving Harry Pearson's maggies and was smitten.

In 1980 I bought a demo SP-6b and D90, driving IMF monitors and stayed happily with these for 10 years until I traded the IMF's for Thiel 3.5's in 1990.  Again I was happy but eventually traded the D90 for a VTL ST-85, a mix of trade-offs.  Eventually I sold the SP-6b when I went to surround sound in the early '90's, but about three years ago bought an old, much used 6a which I had upgraded with new caps and a new tube set.  The magic is back, with an even better bass than the 6b.  I am currently contemplating purchasing a D115mkii to complete my return to ARC.

To answer your question as originally asked, yes older ARC stuff is still good and a great bargain in today's market.  Expect to do some refurbishing, but also expect you will be content for many years.