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...
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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 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.
I had similar questions with my ARC SP-8. In the end I sent it back to ARC to be refurbished, rather than to another specialist to have it modified/changed. My logic was that while an upgraded SP-8 might be an improvement, it simply wasn't going to turn it into a REF 3, SP-17, or LS-25, so in the end it made more sense to me to save up for a really improved model, rather than try to increment my way up. It made sense to me to have aged components replaced with current equivalents (i.e. ARC replaced the old caps with better current caps) but not to make any circuit changes.