Excellent!
This is something else that gets thrown around online without people really understanding why.
As far as recording the DI’s, as long as you leave your Grace and Lynx how they are you’re good. Once the signal has gone through the amp sim, if you are running into other analog modelling plugins, you may want to make sure you aren’t running them too loud or quiet.
Generally its not as critical as it is for a guitar amp as most studio processors are designed for a wide range of levels.
The -18dBFS thing is also not entirely straightforward. -18dBFS refers to a sine wave when calibrating. Its basically saying when a sine wave is -18dBFS, the meters will say its equivalent to an analog signal level being a certain loudness in the plugin. It doesn’t really have anything to do with your actual signal, its relative to a reference level.
Again I have some videos on this that might explain it better:
