+1 on "In-ear monitors are also a good option - used in conjunction with your amp. "
Speaking as a sound man the pickup you use will definitely make a difference. eg, on Thurs I had an American touring band in a church, and the double bassist/drummer used a B-Brand D1(the older model) and that took 5 minutes to eq and get him sat nicely in the monitors. No amp, though they are a quiet band.
Saturday in a small venue with a local band, cheaper DB(plywood) with a Fischmann and I was rolling off loads of low mids to prevent hooting..And that was with only two accoustics and a vocal in the band!
However, in your situation with that many people onstage, you`re hardly ever going to get a proper chance to EQ your bass to the stage you`re on and that goes for "positioning" your amp also, and so you will always struggle.
The in -ears solution should work well, you can experiment with sealed or open style headphone buds or even expand the idea and stick it all through a small mixer with an ambient mic on a stand to blend in the band.You could get away without using a beltpack initially, just use the headphones out from the mixer.
Sounds a faff but once you had it all in a little rack it would throw up in minutes and you may find you don`t need the large amp so much as a cube type monitoring amp.
MM