Lots of players seem interested in lining out their amps to the PA but I don't see a lot of discussion about it. There is more to it than just plugging in. This photo is a channel from my Mackie 808M powered mixer, which is typical of what many bar bands use. It is set up for a line out from a harp amp.
From top to bottom:
-Monitor Send: You definitely want some harp in the monitors if you are using a line out, but not too much. Just enough so you can hear yourself and hear your balance with the band. I have it set halfway here.
-Effects Send: I send nothing to the effects buss. I prefer the sound of the amp by itself or with the FX pedals I use.
-Highs: Roll off about 25%
-Mids: Flat or roll off a bit.
-Lows: Boost about 20 - 25%
-Trim: Start at 0 - unity gain. It depends on many things. Dial it in so you are not clipping.
-Volume: It depends on where you want to sit in the mix.
PAs tend to be bright. If you just plug in and dial in the setting as if it were a vocal channel your sound might be shrill and annoying.
If there is a sound tech ask him/her politely but firmly to set the channel like this. There may be some small adjustments during sound check, but starting out this way will get things moving in the right direction.