Actually, it makes a fine linestage as-is. No way to boost gain, though. Unless you can find a different output tranny. I've been using mine for the PA-10 power amp testing as a linestage. You just use the "HI" output as line level. The nice thing is that the output impedance is under 20 ohms! It can drive anything. And you still have the "LO" for driving headphones.
If you want more inputs, well, that's just a matter of wiring in a bunch of RCAs to a select switch. Then feed that to the board.
You probably don't need more than the 3dB or so gain it provides. A CD player will typically have 2Vrms output. Most power amps reach clipping at 1Vrms. In reality, the linestage provides -6dB. If you have an SS amplifier, the 1/2 percent 2nd order distortion of the CASTANET might just sweeten it up nicely.
jh