das Boot is not being presented as evidence, but as a representation. the engines of a large ship do not quite equal that of a submarine. Subs inherently being built for stealth and silence. My information comes from Jurgen Oesten.
http://uboat.net/men/oesten.htm This was a debate back when we did DBSM-PS for SH2.Somewhere in between very quiet and thunderingly loud lies the correct sound. I believe, based on what should be a fairly reliable source, that the sound is a lot less loud than we all would think it would be. The bottom line is adjust the sound level to where you would like it to be, it is your game.
if you want to hear the engines thumping away because that is how you think it should sound, by all means crank it up
if you want it to be realistic, turn them down.