QUOTE (w1L50n23 @ Sep 25 2013, 18:57) <{POST_SNAPBACK}>
Now, when I went to do my first test recording, I noticed some line noise bouncing the meters on the virtual soundboard (with no turntable connected yet).
Having nothing connected to those high impedance inputs is probably what's causing the noise. Try connecting shorting plugs across the inputs and then check it again.
A general purpose phono-in needs to have a combination of both high impedance and high gain, and that implies an inherent amount of thermal noise that is physically unavoidable (as in an actual physical limit that is unrelated to any implementation details or limitations of the circuitry itself).
For example if you have a 100k input impedance, a gain of 100 and a bandwidth of 30 kHz, then the (physically unavoidable) thermal noise is 100 sqrt(4 * 1.38E-23*298*100E3*30E3), which comes to about 0.8mV. Compared to a typical line level signal of 200mV this is about -48dB.
Even though this is a "physical limitation" with high input impedance, it doesn't mean it cant be overcome. You just need to connect the inputs to something of lower impedance, rather than leaving them floating.