Put simply, the valves colour the sound at all times. The valves have also been known to malfunction which makes the machine unplayable.
The first picture is a kick sound (with 0% valve), picture 2 is the same sound with 100% valve.
This third picture is the same kick with the valves bypassed.
There is a slight reduction in signal level, but this mod will give you a cleaner sound which is unaffected by the valves.