Listening to open-backed headphones and speakers simultaneously?

It will still muffle the loudspeakers.

I have inadvertently left my subwoofers, but not my speakers on.  I don't recommend it.  

No no no.

if you want headphones just sit 3 feet in between them atc’s and point them directly at you.  Your wife will realize you’ve finally lost it and ease up on you.

This is actually fairly interesting....

One of my systems has a surround-field function, connected to near-field monitors. With open backed headphones, imaging and sound-staging abilities would likely improve.

If you don't have this kind of set up, then there's really no point.