Even if the audio in level could be seen as "low", it's should be acceptable in most conditions.
With audio in slider at max, filter in LP, fully opened, master volume at max, on my test machine I have a +3dB between audio in signal level and master output signal level. this value could differ with device output impedance but it should not be too far from this value.
If you have a doubt about you audio in, you can make this test an let me know the result.
Anyway you won't damage anything by using a preamp.