Would like USB ADC w/ input level control

You might want to consider a completely different approach, which would allow you to eliminate the computer and the digital noise it generates from the a/d conversion process.

That would be to purchase a portable digital recorder, that records onto flash memory cards. Those recorders almost invariably provide level setting provisions for their analog inputs, among many other features. You could subsequently copy the digital file(s) to a computer either by connecting the recorder to the computer via USB or Firewire (whichever interface the recorder provides), or alternatively by removing the memory card and inserting it into an inexpensive card reader that is connected to the computer via USB (if the computer does not already include a card reader). Firewire ports, btw, are easily added to most desktop and laptop computers that don't already have one, via plug-in cards, if the recorder you choose provides that kind of interface but not USB.

Here is one such recorder, that is within your price range. Here are many others, covering a wide range of price points.

Regards,
-- Al

You could add line attenuators between the Linto and the Line Streamer when the signal is too hot. You could knock the Linto down to 54 db, but that does require some work. I do not know much about the Line Streamer, but could you take the RCA output from your preamp and feed that into the Line Streamer. That would allow you to use the preamp to attenuate the signal as needed. Not sure of the impedance issues. Hope you come up with something, since I was considering the Line Streamer as an option.

I would imagine there are plenty of professional ADCs with level controls. As to their sound quality, I have no idea. Start by looking at musiciansfriend.com, jandr.com, and bhphoto.com in the pro audio recording sections. Watch out for volume attenuators that discard bits to reduce volume. Stick to analog controls that precede the A-to-D conversion.