Donny,that's a tough one because it depends on what you are comparing,as they are different animals in a way.

The Audya is the king of uncompressed live sound and great styles. It has features like 4 sub outs,S/PDIF,on and on,but is 12-13 yrs old. Like the G70,you will never see it again,except in somewhat lesser reincarnations. Time and technology marches on with improvements seen in all of the SD line,as far as features are concerned.

SD40 is a great module with many uses,such as light weight shows,being able to perform anywhere. I use it for Karaoke,midi's,and style playing and singing. The sound is very good and live,if not quite so much as the Audya

SX900 is great,and a big upgrade from the s970,although a little trickier until I get used to the new system. Since you didn't mention the s970, I won't bother to mention a couple of thing I like better,but it is minor. I wanted a keyboard with speakers and light that could handle the zillions of Yamaha styles better than what I have. For that,I am overjoyed.

At present,my Pa4X will remain my main board,but the SX900,soundwise, is gaining ground. Everyone has their own needs,and mine are now met.

Thanks for asking even if it is not much help I'm afraid.