Our collective problem in providing advice is that there is no such thing as the model you mentioned. We are trying to second guess what you are really considering. I will take a stab at assuming a different model having owned a SP-6C-1. As previously mentioned the SP-3a was the first significant ARC preamp circa mid 70's and ARC actually offers an upgrade of the unit today to more current specs. My guess is that you are looking at it's successor, the SP-6 series. The "C" version came out late in 1981. I owned a SP-6C, later updated to C-1 specs for nearly seventeen years before replacing it with a used SP-9MKIII. Physically, it has a full 19" rack mount face plate which the SP-3 series did not. The knobs were of the current design as opposed to the older large diameter design. As for sound, it was the first ARC preamp to not sound overly warm and tubey. In fact, the mags of that day called it somewhat icy. I liked its cooler character and definition. My SP-9 has much the same character, only much more air, low end impact, and detail. I wish I still had my old unit for my office system.