Welcome! Thanks for posting!
It would be possible to connect the 901 Series VI speakers to an SA-5, but you wouldn't be able to connect the EQ to the amplifier to use with the speakers. The EQ for the 901s is designed to use a TAPE MONITOR connection or a PRE AMP OUT/MAIN AMP IN connection on the receiver or amplifier, which the SA-5 doesn't have. The speakers will still work with an SA-5 you do just lose the functionality of the EQ.
If you want to be able to use the EQ for the 901s and have them be able to be controlled via SoundTouch, it may be worth looking at a 3rd party amplifier or receiver and a SoundTouch Wireless Link Adapter, that you can connect to the amp or receiver. This would allow you to make any 3rd party system that has an AUX input SoundTouch compatible and stream to it over your Wi-Fi network or via Bluetooth.
Let me know if you have any further questions!
Can you recommend any 3rd party amplifiers or receivers that would work best with this speaker system or point me to a current list? I bought these a while back and have not really been able to use them to their fullest extent. I have them hanging in a 50x60 garage and really want the best possible sound while using my current apple devices. I think the soundtouch link sound like a great addition.