tube preamp for Linn/ARC system???

I think your likely first choice is an ARC SP-8; the SP-8 MkI will give you a very tight low end and good midrange and top end, but the SP-8 MkII will give you an extra layer of air and musicalness. One other possiblity is the a ARC SP-3A1 which is a very enjoyable sounding preamp, but I think what you are looking for is a SP-8 MkII. You should be able to buy either of these preamps, listen to it for a year and if you want, sell them for within a $100 of what you paid for them. If you need to drive a low output MC coil cartrdige, you should consider a ARC PH5, EAR 834P, or ARC PH-2 probably in that order (most expensive to least expensive). Alternatively, with some MC cartidges you might be able to make due with an ARC SP-10 (which has lots of gain) which would cost less than a SP-8 plus the PH5 and it is a classic. SP-10s are hard to find, but they are out there; on the other hand, SP-10s eat tubes (and they have lots of them) much faster than SP-8s.

The prudent choice would be a SP-8 MkII plus a pre preamp if you need one for a MC.

If you like the sound of the older ARC SP series, and you have MC cartridges of less than 1mv output, the SP-6 will not work at all unless you also have an MC step-up device. The SP-8 is an incredible performance step up from the SP-6 but it too does not have the gain for MC cartridges. You have to go up to the SP-10 for this and these don't come cheap on the used market.

Fortunately the SP-9 did not have a gain issue with medium to high putput cartridges, but then again, it did not have the harmonic richness, ambience retrieval and 3-dimensionality of these other older models which were designed around MM cartridges. You don't mention if the phono stage or line stage or both in the SP-9 is not working well. If one stage is working fine, and you have an ultimate desire to go with separate line & phono stages, perhaps this would be a good time to start with one of these in conjunction with the working SP-9 stage. However, something like the fully functional Counterpoint 5000 preamp would be worthy of consideration to replace the SP-9 and it would be a huge improvement across the board.

With the warmer sounding Linn LP12 setup and the more analytical Classic 30, you have components on opposite ends here. If you like the ARC sound, I think throwing in older CJ components here would only lead to more complication as they had a softer top-end that was quite a bit different than ARC products of that time.

John