I’ve seen it both ways. The real truth is that this is system dependent and has everything to do with synergy between components.
I was originally of the mind that the best cable should be on the source. However, I have found that possibly the best cable should be the last cable right before the amplifier (which contributes to the last bit of sonic signature character).
It was in another system where we had two cables, one with a higher resolution. It turned out that the higher resolution was best placed between the preamp and the amplifier. When we put higher resolution cable at the source, the overall sound quality dropped and the sound became somewhat smeared from the lo-res cable at the amp.
For those of you who don’t know, the preamp actually has the input signal run through a resistor (input impedance) and then through an op amp or discrete analog circuit with negative feedback gain. This analog circuit essentially "re-generates" the waveform using voltage from the preamp power supply. A preamp will typically have two inverting analog circuits with a volume control (potentiometer or resistor ladder) in between these two circuits (with gain). As long as the input waveform is close to what you need, the "re-generated" waveform is mostly dependent on the preamp power supply and circuit quality. So, the interconnect between the preamp and amplifier may play an even more important part than the interconnect from the DAC/source.
Not all preamps have two analog circuits. There may be some that have a single non-inverting analog circuit, probably with the volume potentiometer before the circuit. This works more as a "buffer circuit".
