First, a classic optical compressor like the LA-2A is a two knob device: Input level and threshold.
A compressor is essentially an automatic volume control that reacts to the amplitude (volume) of the incoming signal.
So compressor isn't taking the harshness out of the top end, like an EQ. What your threshold is set to with your bass is reducing the amplitude of the attack portion of the note. Read on for the explanation.
The threshold control tells the compression circuit how low or high the signal volume has to be to attract the attention of the compression circuit and squash the signal. The lower the threshold, the earlier it 'grabs' the note and squashes down the amplitude of the note attack, hence "less spike."
The higher the threshold, the more signal the compression circuit allows through, so you hear the attack of the note.
If, for example, you wanted more transparent compression, for example on a vocal or acoustic guitar track, you'd set the threshold level at say, -10 db, or thereabouts, and you'd hear the note attack.
If you want to squash down the attack of the note to reduce the spike caused by the note attack, you'd set the level lower, like maybe -30 db, or so.
Thus for vocals, you might want to allow the transients through and only engage the circuit at a higher volume level. If you want that Nashville chicken pickin' thing, you might want to set the threshold lower.
Etc. Carry on!
