Is it allowed to pan low frequency instruments? It is extremely useful doing that with guitars, vocals and cymbals, but what if I had low-frequency? How can I distinguish the similar low frequency sounds? The mix is a mess!
For release on old-fashioned vinyl, the answer is no. Low frequency sounds should be panned dead center. The reason for this is that differences between the channels cause the groove to become shallow. This is a particular problem in the low frequency end of the spectrum and it is possible for the groove to become so shallow that it almost disappears and the stylus has no path to follow. If you do pan low frequency instruments left or right, then the cutting engineer will probably simply sum the bass into mono. Although vinyl is very much a minority sport these days, the idea that bass should be panned center persists.
For any digital format, technically you can pan wherever you like. The only restriction is that if you like your bass to be loud, then panning it to one side will result in a 6 dB loss in the potential maximum level that could have been achieved if it were reproduced by both the left and right speakers.
Gaining clarity in the bass end of a recording is another matter.
Rule number one is to to cut the bass below around 100 Hz for any instrument or sound source that has not been specifically included in the mix for its bass content. Anything else is just mush, and should be filtered out.
In many cases, that will be enough to provide clarity in the bass. However if you wish to combine several bass instruments, you need to carefully EQ them so that each has its own little space in the frequency spectrum.
For example you could EQ up a kick drum around 80 Hz, then cut the bass guitar slightly in this region while boosting it in the 150 Hz to 200 Hz zone. These are not hard and fast figures, just instances. It would be up to you to experiment.
One problem is that most monitor speakers have a resonance in the low end that confuses the bass. Other than using electrostatic loudspeakers, there is little you can do about this. The best solution is to have two or three different pairs of monitors. Each will have their own faults, but with a little effort you should be able to come up with a balanced mix that will sound good on them all.