Really enjoyed the lesson. I have question about tritone substitution.
Most of the theory that I've read on this subject says that a dominant 7 chord can be replaced with another dominant 7 chord which is a tritone interval away from the original. However, in the Amazing Grace arrangement, you replace an D7 and and A7 with an Abmaj7(#11) and an Ebmaj7.
They sound great but I was just wondering if there is a rule that the new chord substituted for the dominant 7 should also be a dominant 7. In other words, when can the new chord be something other than a dominant 7?
Hope that makes sense.