For what it's worth, I did some detailed research on passive preamp stepped attenuators a while back for a different project. The conclusion I came to was that the current products available from Khozmo were the absolute superior in any passive preamp. Since you are running unbalanced RCA interconnects in your system (based on Exposure 17), a Khozmo Passive Preamp Dual Mono with input selector (with what looks like 3 sources) is $558 shipped to US with all options (such as Vishay nude Zfoil and Takman REX shunt resistors). They can be built with an impedance of anywhere from 10k all to way up to 200k, depending on the best impedance for your system. That being said, this is truly passive and not introducing any sonic signature at all. You are entirely dependent on the sonic signature of your turntable and amp. Since your using a turntable, this might be an option. I would not normally recommend a passive preamp for a digital DAC, unless you were using a tube-based DAC or a DAC with a nice Class A based output stage.
That being said, the amp may not have enough gain to work well with a passive preamp. Some amps can have a gain as low as 26db. While this can, obviously, work with passive preamp, usually people like to have an amplifier with a gain of 30db or more if they are using a passive preamp. I couldn't find the gain for your Dual Reg 4 amp.
Since my experience and skillset is all on the solid-state side, I will refrain from giving a strong recommendation. However, I did do a little bit of research into ARC in the last 8 months (just for the heck of it). What I found is that ARC usually seems to be more neutral and fast for a tube device (definitely not slow or laid back like some tubes). However, I did find that you could not try different tubes because the tube choice of ARC can only be obtain from one Russian manufacturer. This won't be like swapping out 12ax7 tubes to get your preferred signature. I will say that ARC puts a --tremendous-- amount of work into designing, testing and fine-tuning their designs. All their equipment is hand-built and hand-soldered. Their board layout and design are excellent. All board traces are very thick and heavy for good current transfer in signal and power supply. That being said, they are designing towards their own idea of the "best sonic signature", which may or may not be what your idea is. Their listening room is also setup a certain way (they use a LOT of QRD diffusion panels on the front/side walls which may affect the end-result sound).
If I was into tubes, I would probably heavily consider an ARC preamp. For example, there's an LS26 on audiogon now for $2489
