Would it be possible to use a USB sound card (e.g. Creative Soundblaster X-Fi Surround 5.1 Pro USB Audio System with THX SB1095) as a volume control for the line-in signal without connecting it to a PC? I don't know if the line-in just passes through without attenuation by default, or if software is required.
It isn't impossible for a soundcard to have a line level control that works pre ADC, but it is uncommon. Any control on the line-in of most such cards is post ADC. That is, it works in the digital domain, after the signal is already sampled. This is essentially useless vis a vis the line level input, it can't help anything about a signal that originates in the analogue domain. In some cases it can be useful when multiple digital input signals are mixed, to prevent clipping.
A microphone input may have analogue gain or input level controls but these are useless for a line level signal. Putting a line level signal through a microphone preamp is not going to produce anything very nice. So, even if one could come up with some way to control the soundcard hardware outside a computer (software controls the hardware), using it that way is very unlikely to be satisfactory.
There are USB "interfaces" that include microphone preamps, mixers, and sometimes other hardware. Line level input adjustments are still uncommon, but may be included. some of these units can function quite well in stand-alone mode; they have analogue out as well as USB out.