If you have a solid state source like a CD player, quite often it has a coupling cap at its output
What Ralph mentioned about coupling caps above is not relevant these days, as nearly all solid state sources have been direct-coupled for years, only tubed sources may have this problem as they have a caps on the output.
But below tubed preamps have this problem at the other end of the system with some of today's low input impedance amps.
All is not good a match with 99% of tube preamps with low input impedance amps these days, many Class-D's, some First Watt's, some Pass Lab's and many others.
The coupling cap that's needed on the output of 99% of tube preamps can be too small for these amps I mentioned, which can be as low as 10kohm input impedance.
This is not a good match and will roll off the bass very early, to the point of owners saying this "tube pre has little or no bass".
To fix this one can change the cap for a much larger uF one which can be a fitting issue in itself or revert to a not so good sounding bi-polar electrolytic cap which will be small
Cheers George