Can line 6 lowdown 110 play side by side with a drummer in a theatre?? and how many dBs is this amp??
That is hard to say. That amp has 75 watts, but it is all going through a single 10" speaker. I think you would be getting a lot of distortion from the speaker if you try to keep up with a drummer.
In a word, no.
IIRC, that amplifier has both a Preamp Out and a Direct Out feature, so you could use it as a Direct Box and go straight into the board of the P.A. system. Then you would have no problems keeping up.
Do you already have the amp or are you considering buying one? If former, you could just use the DI feature (your sound goes to the PA) and use the amp as a monitor. If latter, don't buy it. Get something more powerful. Go used and you can find better stuff for lower price.
I believe that Line 6 has discontinued all of their bass amplifiers. I do not know if that means they are planning to launch a new series, but there are no bass amplifiers listed on their website right now.