Ok, here's another question

F7-Bb

I'm playing a B6-Bb6, voiced as B-D#-G#-B over the F7.

Now, F-Ab-B-Eb is Fm7b5 - which also to my ears doesn't sound bad going to

the Bb.

I realize that B7 is the V7sub for F7. Is B6 also, or B6/9, etc.

Or am I hearing the B6 as "ok" just because it's a chromatic approach to
the 
Bb, and not really a substitution? Or are there Major substitution chords?

Steve