My question - would my set up benefit from a cab sim rather than just a pre amp?
The answer depends on specifically what you think bass should sound like. If you prefer the sound of an amp and cab, then a cab SIM may be very useful. However it's not a matter of buying a random cab SIM and calling it done. You have to find a SIM that is capable of producing the sound and response
you want. My impression is there seems to be a pretty high level of disappointment in available cab SIMs. You may find this recent thread informative:
https://www.talkbass.com/threads/are-all-cab-irs-terrible.1685670/
Ideally you would know how the mixer is configured, and understand the ramifications of every adjustment you ask for or make. But not everyone has the interest in learning the ins and outs of how something as complex as an audio system works. The following may seem confusing and long. I suggest reading through it; not necessarily to fully understand, but at least to get some idea of the level of complexity you may be dealing with.
There is pretty much an infinite number of ways an IEM system can be implemented, and most of the options are somewhat messy. By messy I mean they probably don't work they you think and they are probably not capable of giving everyone exactly what the want. In order to avoid unnecessary frustration, you need realistic expectations. Live sound rarely approaches ideal. It's often more about trying to achieve the best possible compromise, which often falls far sort of desired results. Sorry this is reality IMHO.
The purest implementation of IEMs involves splitting the signal into separate monitor and front of house (FOH) mixers. This option allows the bass to be optimized for both the house and monitor mixes. This level of system is fairly rare for most of us. @s0c9 has provide some very good information on this type of system.
Some digital mixers can be split to essentially function as two separate mixers. For example I have run Yamaha LS9s set up so channels 1-32 were for the FOH mix on Fader level A. The inputs were dual assigned to channels 33-64 which were used for monitor mixes on Fader level B. IME, this sort of setup is confusing and labor intensive. It can be extremely useful, but I would only recommend it if you always use the same audio tech and PA.
The most common approach is to use one mixer for both FOH and monitors. FOH is run on the main stereo buss and monitors are run off of Aux Sends. A big limiting factor with this sort of setup is there is only one set of controls on each channel. The controls can be used to dial in the bass for the FOH mix, or they can be used to dial in the bass for the monitor mixes. For the most part the only channels controls that are separate for FOH and monitor mixes is the level controls.
IMHO if you view yourself as a professional, you should probably prioritize what the audience hears to some extent, but you need a tone you can work with. Rather than trying to direct the audio tech how to dial in a sound you can use, IMHO it can be better if you have some processing at your fingertips that allows you to get a usable sound. This is where a preamp DI or cab SIM could be useful. Keep in mind any modification you make to your tone will impact every mix that has bass in it. So try to get your sound dialed in early and then lock it down during the show. Also, if your tone settings are too extreme, you may need to work out a compromise with the audio tech.
If you need a level bump during the show, its best if the change is made on the mixer in way that does not impact the main mix or other monitor mixes. In other words, ideally you should not bump the level you are sending to FOH because it can cause a lot of problems. Some digital boards allow you to use a phone app to adjust send levels for your IEM mix, so basically you can adjust the balance of sound sources in your personal mix without impacting any other mixes. Some systems do not have this functionality, so you have to ask the audio tech to make adjustments for you.
The way a mixer channel is configured can vary considerably. There is variation from one mixer to the next, and different audio techs may use different configurations based on their philosophy and work flow preferences. Usually the signal that goes to FOH flows through the entire channel strip. But many mixers have a range of selectable pick-off points for the Aux Sends. The most basic setting is Pre VS Post fader. For monitors the Aux Sends should normally be set to pre fader. If they are set to post fader, then any balance changes the audio tech makes to the main mix will translate over into the monitor mixes.
Another important option is Pre VS Post EQ. If the aux send is set to post EQ then any changes to the channel tone controls will translate to both the monitor mixes and the FOH mix. If the aux send is set to pre EQ, then changes to the tone controls are applied to signal that goes to the FOH mix, but the signal going to the monitors is not impacted. Which is better depends on the situation and preferred functionality. Philosophically I think the channel controls should be used to make the house mix sound as good as possible. Sometimes the required adjustments can be quite extreme and severely degrade what is heard in the monitor mixes. So using Pre EQ for the monitor mixes can definitely be useful.
Basic systems tend to have less features and controls. With an entry-level analog board, you may have gain and a few tone controls on the channels. With a more advanced system, you may get variable HPFs, compression, 4 bands of parametric EQ, and effects for each channel. The system may also have global compression and advanced EQ options on mix busses/output. FYI, "global" controls effect an entire mix rather than an individual channel or sound source.
This is sort of scratching the surface. Hopefully this is enough to give you a better idea of the complexity you may be dealing with.
Good luck!
