How is VE-RD1 connected to A70 compared when it's in A90? Is there any sound quality difference?

Danb,
Basically the sound will be the same but, there are some issues that you should look into:

* The output of the A-90 is the balanced type and the A-70 is not. This may or may not a issue for you.

* The different keyboard response can change the connection between you and the piano sound.