I had a long, love-hate relationship with the ARC SP-10 mk ii. I bought mine used, from somebody who had to have the new SP 11, and knowing what the SP 10 was capable of, I jumped on it. I had it sent back to ARC to have it gone over (at my cost) and have it retubed. I had a devil of a time finding quiet tubes, and I don't think it was the tubes that were causing the microphony, but how they were mounted. The unit went back and forth to ARC quite a few times; I bought new at the time gel type sockets (of a type similar to those used by Allnic) and ARC was willing to install them- they were also interested in the outcome. They didn't help. I had bags of 6dj8s that I would swap into the thing; I did have the factory at one point hot rod the unit to bypass more of the switches. It was a very good sounding preamp, with a sweet phono stage, but still of the old school ARC sound- not quite syrupy, but tending toward warm, rather than that more bleached sound of some of the later ARC stuff that sounded leaner. Once I got stopped using the Crosby Quad, which seemed to mask some of the low level noise of the preamp, and started using horn speakers of high efficiency, the preamp was simply too noisy to use.
It was sort of my peak with ARC-- though I had several of their amps over the years (and still have my first one, a Dual 75a), I migrated away from ARC. Not due to any negative attitude on my part- I maintained a long cordial relationship with the parts department for decades.
When I got a Lamm L2 Reference line stage back in 2007 or so, I had SP-10 flashbacks- the unit was very similar in lay out, though the design and sound very far different. Classic old school preamp.
