Pete, my knowledge isn't hands-on, but rather from reading past posts here. All I've ever done is replace a pump and the belts on my '87 Maytag A712.
You described a knocking sound as opposed to a banging sound, so the original problem could have been something as simple as a worn belt, but I have a feeling you checked that. There was a recent thread that contained a video clip of a Maytag with the motor going spastic on its glides and making a loud ka-chunk noise that sped up as the RPM increased when the washer went into spin. The cause was just a chewed up belt.
The tub springs usually last the life of the machine, but you never know. There's no shortage of Maytag experts here who are far more qualified than I am and can do more than just speculate. Give this thread some time and one of them will likely contribute their knowledge. In the meantime, you can do a search here that could turn up a thread that speaks to your issue.
One thing is for sure. Your A506 has effectively disproved the final post in the thread that Bob linked to above.