Q: If I filter out low frequencies, can I get them back again later?
You are correct in thinking that it is often useful to get rid of low frequencies (below 100 Hz) on everything but your bass instruments.
This can increase the clarity of the mix tremendously and is very easy to do.
But what if you do change your mind later? Are the frequencies you have filtered out gone for ever, or can you boost them back up again?
Well imagine that you have applied a 12 decibel per octave (12 dB/octave) low-cut filter at 100 Hz. This means that the level at 50 Hz is 12 dB down; at 25 Hz it is 24 dB down. Below that usually doesn't matter too much.
Let's suppose that frequencies below 50 Hz are already 30 dB or so below peak level (-30 dBFS), as they easily could be.
So after filtering, your signal around 50 Hz is down to -42 dBFS, and at 25 Hz down to -54 dBFS.
So these frequencies have not disappeared, but they are now getting quite close to the level of the noise that is inherent in any recording system.
So, if you boost these low frequencies back up, you will also boost the noise.
But that need not put you off. There are many situations where noise is present, but is masked by other things going on in the signal.
If you had used a 24 dB/octave filter at 200 Hz, then things would be different...
If you started at -30 dBFS, then 100 Hz would now be -54 dBFS, 50 Hz is -78 dBFS, 25 Hz is -102 dBFS.
So your lowest frequencies are now effectively gone.
So in summary, you should be aware that although filtering can often be undone to an extent, extreme filtering can be forever.