Audio Research Sp-6a-Should it be modified?

it depends on the price and the level of performance upgrade. The 6a is pretty good without an upgrade, maybe the money should be put elsewhere...

The best bang for the buck, and relatively simple to do, would be to replace the Wondercap metalized coupling caps with higher quality caps.

I would upgrade the parts but leave the circuit the same. The caps are old and much better ones are now available. Resistors have also improved. I would stick to original values for both. Modifications to the original circuit will produce a different sound than the original which you may or may not like better. The first approach is likely to give a significant improvement without altering the basic nature of the sound.

I recommend that you ask Steve Huntley at Great Northern Sound Co. He's honest and his work is exceptional.

I got my first ARC pre-amp in 1989- an SP-6b. A buddy of mine who was experienced and competent with modifying ARC gear did a nice upgrade for me. He replaced the film capacitors with WonderCaps, changed the resistors to metal film, new trim pots on the tube voltage bias circuits, Litz wiring throughout and gold RCA jacks. I enjoyed that preamp up through 2003 and only changed so I could experiment with low output MC cartridges.