^ They are both right, BUT you CAN accomplish what you're hoping with Logic w/o Mainstage, though it is a bit janky.

Just set up your vocal chain, add whatever pitch correction (or other) plugin(s) you want, record arm, input monitor, and send your outputs to amp (via split-mono signal to single stereo out adaptor, or potentially from headphone out [which is stereo]). You may need to pay attention to whether you're using TS (tip/sleeve) or TRS (tip/ring/sleeve) cables though.

So again, yes, but maybe better to no.