Well, the rule of thumb in orchestration (and EDM arrangement, for that matter…) is to stick with unison and octaves only down there. Bass is difficult to hear (especially if you don’t have massive speakers and a well treated room), and there isn’t much space to work with, so you need to keep it very clean and simple, compared to the higher frequency ranges.
Of course, you could layer different articulations in the bass, just like everywhere else, but (even more than in the higher ranges) you need to think about it as layering for sound design, rather than on a musical level. For example, brass staccatos + woodwind and strings longs to create a beefy sound with bright, distinct attacks - but they need to play the exact same notes, or it will just blur everything.
Indeed, one can use EQ to shape the sounds to fit together in the mix, but if we’re talking traditional orchestra, that can only be used for subtle tweaks, if you want realistic results. Of course, if there are synths and sound design all over the place, anything goes, but even then, it’s usually better and easier to do the basic “mix” on the arrangement/orchestration level, and by selecting sounds that actually go together without radical processing.
As for stereo field and panning, you should generally not mess with that, at least when using wet samples, as manipulating the reverb tail can create pretty weird results. The whole point of using wet samples all recorded in situ, in the same hall, is that the instruments really sounds like they are where they’re supposed to be, and that is ruined if pan/balance/width is altered per track in the mix.
That said, you probably should make the very lowest bass more or less mono, but that should be done with a suitable plugin (an EQ that can work in M+S mode, for example), either on the master, or on the problematic tracks only. The change should barely be audible (and literally won’t be on a 2.1 system), but if you play “wide bass” on stereo (non sub) speakers or headphones, it will sound and feel very weird.