One of our customers was upset because he loved the sound of our phono preamplifier directly into his power amplifier but then not so much when he inserted his preamp between the phono stage and his amp.
His question, naturally, was this: if my preamp sounds great by itself and the phono stage sounds great by itself why do the two not work so well together?
It's easy to ignore the cumulative degradation of equipment where each piece in the chain adds a bit of flavor not natural to the source - but preamps may be the worst of the lot.
A preamp consists of three elements: an input switch that selects the desired input, a volume control that provides its namesake and an output amplification stage that provides the gain to drive the power amplifier. Problem is, while the first two elements in the chain are needed the third is not and usually gets in the way.
The vast majority of sources you connect to a preamp have enough output to drive your power amplifier directly, in fact, many have more than enough. Preamps reduce that volume level of the source only to re-amplify it back up to match what the amp wants. This process can only cause harm to the purity of the signal.
It's natural to want a preamplifier in the mix because we all have multiple inputs we need to select from and, at a minimum, we all need to turn down the volume. If every manufacturer built power amps with multiple inputs and a volume control there's be no need for any of this but we don't.
'Tis a shame.