I use stereo iems in two bands, and mono iems in another band. It’s not necessary, but I really enjoy stereo much more when available. It creates a more realistic stage soundscape and builds more isolation between instruments when used effectively.
If you aren’t running wireless for your bass, I would NOT run wireless for your iems. One main reason is that wired just has better sound quality, and another reason is wired avoids any static/noise/interference/drop outs that are too common with wireless systems. Finally, wired is exponentially cheaper, so for me wired is a no-brainer unless I ever decide to run my bass wireless. I followed the same path as @drpepper and built my own cable/snake that had a 1/4” cable for my bass paired with a 1/8” headphone extension cable. I’ve since bought an Ernie ball iem/guitar cable as it’s even more robust and my personally built cable is now a backup. I run the behringer P-1 as my pack. I mostly run it stereo, but I can just plug in one aux send and push the stereo/mono button to switch to mono. The pack is mounted to my effects pedal board and runs 9v power off my pedal board power supply. Super simple and reliable. If you don’t have a pedal board you could set the pack on your cab or amp, or wherever you plug your bass in.
Unless you are running your bass wireless, I HIGHLY recommend this type of wired iem setup. It feels just like running bass with a normal cable.
Here’s what my system would cost you today;
Ernie Ball iem/guitar cable; $75
Behringer P1 headphone amp/pack; $50
Whatever ear buds you want; $50-1,500
Here’s a view of the front of my pedal board. My iem pack is mounted to the front of the board. My Ernie ball bass/iem cable is plugged into the pack and tuner.
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