I can't say I know much about BIAB, but it should pick up your MIDI device from the interface. If you are bypassing the interface, should work as is and only your analog tracking will change (ASIO select interface, Lexicon select) should be the only real change.

MIDI is handled separately than audio. However, ASIO will be used when the sound is returned from the MIDI track when monitoring.

MIDI is more or less a service from the OS. ASIO is used to increase performance and bypass many of the OS'es overhead and talk more directly to the hardware (and it is written for that hardware).

Hoped I helped and not confused.

You should download the last drivers for the Lexicon. It should include the asio drivers.
As for Midi, the lexicon doesn't have a midi port.. but you can still use the internal midi commandes and notation within the DAW.
If any day you buy a midi controler, keyb drum or just pads, make sure they come with a usb connection that will transmit midi information.
Just in case you don't know, midi is not sound. it's only instructions that help DAWs and Controlers to communicate to each other and virtual instruments. . .

pcrecord and DM60, thanks.
So, if I understand correctly, I would leave my MIDI settings the same , choose my interface as default I/O for my computer,and only change to ASIO within the DAW.
My Midi sounds will still play back through the interface, which will handle it through ASIO, correct?

Yes... pending any weird ball-bounces, that should be the way it works - although you should also do what PC mentioned, and visit Lexicon's site to make sure that the drivers that come with the Lexicon are up to date.
It's not uncommon for some of the devices to have been sitting in inventory for a period of time long enough for the manufacturer to have released driver updates over the ones that were originally included with the one you purchased - so just make sure you are using the most recent drivers.

To add to Donny's post, I rarely bother with the CD for drivers, I just go the manufacturers website and download the latest. Most likely the drivers on the CD are "first" iterations and haven't fully field tested. There are usually changes after initial release.

Thanks, guys. I'll definitely do so.
Post Office says it was delivered to my parcel locker a few minutes ago. Yehaw!