It's all up to yourself and your personal preferences.
To the biological/perceptive question: You can clearly identify 2 different tones in 3 Hz steps up to a minimum of 14-16 Hz. Below that, until 8Hz you have a resonance-frequency, which gives you a humming sound, just like the deep roaring of an engine or echoing of an explosion or thunder.
Little hearing test:
http://www.dogstar.dantimax.dk/acoustic/440.mp3
http://www.dogstar.dantimax.dk/acoustic/443.mp3
Play them after another and you
should note the difference.
Frequency in IL2 surely not dampened at <50Hz and it indeed makes a difference, as it is the difference of a Merlin Engine and a moped.
The problem is, though, that most onboard-soundchips don't display these sounds and their spectrum very well. But then, you surely wouldn't expect to play IL2-1946 with all details to max on an onboard-graphics-card, either.
And if you really think that a soundcard only makes a difference with a 5.1 soundsystem and above, you really should spend some 20 bucks for a soundblaster to hear the difference. As KG26_Alpha said, you can get a lot more even out of the small Medusa-Speakers by using the CMSS 3D spatial sound and the 24bit crystalizer, bass-rerouting and THX-5.1-distance- and speaker-calibration. With an X-Fi you can also use a stereo-headset with spatial sound that comes pretty close to full 5.1, when using a decent headset. Surely speakers don't affect the soundfiles, but they can represent them or not.
It's surely not only crappy boxes, as they only can mirror the sounds they are presented. But you wouldn't fit 22" tires to a Fiat Cinquicento either and expect it to break a Formula1 record.