Strictly personal opinion based on owning back in the day both the D70MKII and the following ARC preamps:
SP-8: later versions, would be first choice
SP-9: MKII or MKIII, would be second choice
Although I have not owned one, I have heard the SP-6E many times as well, always with excellent results.
The ARC SP8 is an all tube preamp (8 tubes total). The SP9 and SP14 were hybrids (2 tubes for SP9, 1 for SP14).
I have owned all of these and really preferred the SP9 and SP14 to the SP8.
IMHO, the SP9MKII or SP9MKIII are excellent choices. Tube rolling is also very inexpensive and easy with these preamps.
I have owned five different AR preamps....., and I loved them all except the SP9 Mkiii. It was too thin sounding and totally lacked guts and body compared to the others... I sold it after owning it for only a week.
.... Other AR preamps that I have owned
LS 10, Ref 1, Ref 2 mk2 and Ref 3
Cegtv1, you might try and listen to a CAT preamp. I use a SL1 Sig with rolled in tubes with my ARC D70Mk2 and IMHO it is a great match-up. The ARC is good enough to reveal most of the faults of up stream gear and a great tube preamp really lets the ARC do its thing.
I have also owned the Sp-9 mk1 and mk2 and hated both units( went to SP-11 )..Much too thin and bleached out..I have owned many Audio Research pieces and the SP-9 (mk1 and 2 ) was by far the worst of there products I have ever owned...Spend the extra $$ and get a SP-11..You'll be a much happier person once you do...........
i want to thank everyone for input, i will now decide what to do
My experience is simillar to Thorman`s, I owned the SP-9MK2 for about 15 months(durning the mid 1990s). I found it to be rather thin. flat and electronic in character, I replaced it with a Quicksilver full function(all tube) preamp and it was a much better preamp, more dynamic,vivid,better tone,harmonics and more natural sound. I`d consider something else in the AR line.
Any version in the SP6 and SP8 family.
I would avoid hybrids. They just don't sound right to me.
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.
Well I am in error. After looking up the circuit diagrams the SP-8 does not have a tube rectifier, it has a tube voltage regulator which I still prefer, easier on the tubes. Sorry for the confusion. This is what happens when you don't check your facts. My bad.