ST Bags
I do like the Sanitaire ST bag system. I have the "dirt tube and overmold assembly" for three of my Sanitaire vacuums at home. For blue line Sanitaires and Eurekas, the dirt tube simply installs over the flexible F&G tube at the bottom of the vacuum bag. My F&G work vacuum, with the latch style coupling between the vacuum base and the bag, won't accept the dirt tube with out replacing the outer bag. It is similar to the OP's sc684. To change it over to the ST system, he would need to install a different outer bag with the screw down flange to attach it to the base of the vacuum. It's a pretty simple task.
For my work vacuum, a sc679 with a black base, I simply use a latch coupling style black F&G bag assembly with cheap F&G bags. It's still cleaner than a shakeout bag. I get 20 Eureka brand F&G bags for $15 at Amazon, so it doesn't cost me much to replace partially filled bags at the end of the day. That vacuum doesn't need the superior filtration or airflow of the ST bags. With a clean F&G bag, it still does quite well on business carpet.
My s635, a 5 amp blue line Sanitaire, works very well with ST bags. I love using that vacuum at home with ST bags.