I think you'll have a hard time finding an analog volume control with noise levels that low. Analog passive volume controls are usually potentiometers or switched resistor ladders, and each of those imposes resistor noise. The any additional circuitry will impose more resistor noise. In theory, that could be as low as -125 dB or thereabouts for the audio band, but in practice getting anywhere near that is not at all easy, particularly if you want that device to do ANYTHING else.
My B&K preamplifier has a line-amp bypass switch, but it still goes through the main volume pot and also the balance pot. Those pots are high-quality Noble potentiometers, but at anything less than full volume will impart enough noise to pull the SINAD down below your threshold.
But there are other considerations, too. Firstly, if you are listening on headphones, your headphone amp probably already has a volume control, but headphone amps can be made cleaner than speaker amps because the currents are much lower.
Another consideration is what you can actually hear. Let's say you are running the amp without volume control, which means the sound at one meter might be 110 dB SPL. I can't think of anywhere a person might live as having less than about 30 dB SPL of ambient noise, and for most real houses it's probably closer to 40. That means even at full power into real speakers, the signal envelope is 70-80 dB. Even if you want your electronics to have noise levels a full 10 dB under the ambient noise floor, a preamp with a SINAD at full volume of 100 dB is abundant. But that means that at one tenth of full power, the SINAD will be 90 dB--still 10 dB greater than the signal window most people live with whether they realize it or not.
I had the same issue as you. If I'm using this miracle-DAC with its SINAD of 120 dB, and an amp with a SINAD of 120 dB, then why would I want to degrade that using a preamp with a SINAD of 100 dB? Here's why--I didn't need and can't use the high SINAD of the DAC and amp in a real listening environment, and the preamp doesn't degrade anything that can be heard in a real home. Of course, the speakers themselves are, at many frequencies, barely capable of keeping distortion below -40 dB, let alone 100 dB. In a moment of clarity, I realized that my old B&K preamp still provides a signal envelope that encompasses the dynamic range of CDs, and there's no way in the world I can hear the difference between a CD (at 16 bits--96 dB) and even 20 bits (at 120 dB), even without considering the noise floor of my living room.
So, get something you can afford and don't worry about it.
Rick "contentment is a decision" Denney
