Bought a Carver TFM amp in the mid 90s. Sounded great with CD player going into it directly. I used the gain knobs on the front panel for each channel as volume knobs. Sounded clean.
Sometime around 2008 I bought a new NAD preamp...entry level stuff @ $700. I was expecting to lose a little resolution going into the Carver. Sounded soooo much better. More body, more bass...it had this softness and smoothness. It did not make sense at the time.
I had to turn the gain knobs on the amp all the way up to max (as recommended by the Carver manual) to use a preamp. I thought it was going to blow up...but it sounded beautiful.
The class A gain stages in preamps are designed to handle very delicate signals much better than the gain stages in your average amp.