Hi i agree with this
With the L3m in electric guitar mode, you might try using a PRE amp model on the HD500 (no mic/cab selection) and the physical switch set to Amp. In this case the L3m emulates the cab; the setup is similar to using just the power section of a regular guitar amp (the HD500 is the preamp) so you don't need any mic because you're listening to the cab output directly. Try that setup and compare with the former.