discreet devices are typically class a. Like a single FET used as a preamp. Discreet devices are most linear in a small region of their conductance curve. Class-a biases the signal to center in the linear range. It takes some design to get this right.
Most preamps use IC's operational amps (op-amp). They can be very quiet, and have lots of headroom and very linear at all levels and are easy to design.
In either case it's all marketing. Go by the measured specs. (
that's right - you can only trust your ears if you have hours and hours and hours of experience with the gear.)
You want low noise, high headroom, and impedance's that match what you're plugging in. You're just trying to get the analogue signal into the digital domain where noise and headroom aren't a problem.
