As I've said, before (I've been told this is covered by Levine's theory
book, but I'm still waiting for mine to arrive), this all has to do with the
melodic minor. As there are no avoid notes on this scale, you can use
virtualy any of its modes in substitution of any other. They are
interchangeable.
Relevant modes may be :
I m (6, maj7,9,11) - melodic or jazz minor
bIII maj7/6 (#5,9, #11) - lydian with #5
IV 7 (9,#11,13) - lydian dominant
VI m7 (b5,9,11,b13) - locrian with maj9
VII 7 (b5/#5, b9/#9) or VII 7 alt - altered dominant
For instance when you have a minor 6th chord as in a plagal cadence, you can
subs it by a 7(#11) - lydian dominant - a perfect fourth above
(correspondance between the I and IV degrees). An altered 7 by a m6 half
step above (between VII and I). A m7(b5) can be replaced by m6 a major 6th
below (between I and VI) etc.
The improvisation scale is allways the melodic minor of the respective I
(one).
Notice, that even the tritone sub is present between the VII and the IV, and
is this correspondance to the IV that imo is responsible for the statement
that the scale of the subV7 is the lydian dominant scale, with a #11!
In the particular case of plagal Em6 substituted for A(9,#11,13) thats why
other extensions/alterations of the A7 won't work (they would go outside the
melodic E scale, thus making the substitution invalid).
Pedro