this is maybe a numb question, but is there any program that can generate sound at specific frequencies to test which frequencies are you still able to hear? I thought it can be useful while setting the lowpass filter to save you some bits that can be rather used for frequencies that you are able hear.. Does it make any sense at all?

foobar2000 has a built in tone generator. Adobe Audition is also able to generate a frequency sweep, and I'm sure most other audio editors are too.

Beware though. Testing your hearing on a frequency sweep is not the same as testing it on actual music, and will probably yield different results. You should try instead to rip a track from a CD that you are familiar with and progressively adjust the lowpass downwards, and note at wich point it starts to sound different to you. Verify your findings with ABX testing.

you could also try these
http://www.hifiaktiv.at/diverses/testfrequenzen.htm

Listening to test tones will not tell you where to set the lowpass value. This is because pure high frequency signals are easier to hear on their own than when they are masked by other frequencies (as in typical music).

For example, I can hear a pure 17.5KHz test tone but I simply cannot differentiate between a 16KHz lowpass music track and a full 22KHz bandwidth one. Those 17.5Khz tones are there in the full bandwidth clip but I just can't perceive them because other frequencies, to put it in simple terms, are more apparent.