Effects Loop Question

["Jesse Kudler" <jkudler@wesleyan.edu>]

This is probably a naive question, but here goes:

I've never used my amp's effects loop, but I want to try running my Vortex
through it.  The manual says the nominal output is -2dBu, but my amp only
has settings for +2dBv, -11dBv, and -24dBv.  Do I just use the closest?
Will this be okay?  Are "u" and "v" the same here?

Also, is there a significant difference between going from guitar to effect
to amp and using the amp's effect loop?

Thanks in advance for your help.

-Jesse