for me, the following are essential gears:
1) DBX driverack pa/pa plus or even higher models such the 260, 480 or 4800 which can serve as the 'complete processor' between your mixer and the amps.
driverack pa plus
2) DBX 1231 or 231 dual 31 band graphic equalizers if you opt for added 'quick fix' just in case you decide to equalize your f.o.h. and typical monitors.
1231
231
3) DBX 1046 quad compressor/limiter for selected channel or even group inserts if available
1046