audio research

Paul, I just noticed that you want to stay within the ARC line for your preamp.
The SP10 was the preamp that was available at the time of the D70's. I think this is a great preamp if you can find one today. It is hard on tubes BUT has an excellent phono section and IMO is superior to the SP11, 9 or 8.
Like it almost as much as my CAT..:0)

There was much controversy on the SP-9 with conflicting reviews when new. I auditioned one from a dealer and thought it sounded wonderful. The one I bought was sealed in the box and did not sound nearly as good, so I returned it.

I never followed up on differences between different ARC versions but my recommendation would be never buy a SP-9 without a trial audition in your system.

I prefer the SP-8 over the SP-9. The SP-8 can be notably improved by replacing the Rel-Cap coupling capacitors and bypassing the opto-coupler in the mute control.

Audio Research SP-8 MKII is the match for your D-70. There is a big difference between an SP-8 and SP-8 MKII.

I would not go with the SP-9 unless it was a MKIII version. The SP-9 had issues with crosstalk and manifests itself as a lack of space between instruments, as well as a somewhat gray back-ground instead of black. Old f.e.t technology. Instead I would opt for the SP-8, the version with the tube rectifier. That was manufactured around the same time as the D-70 MKII. If you can afford it the SP-10 would be my choice. It is more open, and it offers greater detail. It's phono stage is quite a bit better then the SP-8 providing you can keep it in low noise tubes which now a days is not a problem. If its stock expect to recap the unit. There are a few capacitors in the power supply that need to be replaced. Also, the voltage regulator stages will need to be checked for noise.