So why did Jon step to the background for Edge Of Thorns?
He was obviously very involved given that according to wiki, he played just about every instrument on Handful Of Rain?
Hope you don't mind if I drop in to comment on this
Jon blew his voice out during the Streets tour. Toward the end, his voice started to give, and he would be hacking up blood on the side of the stage during and after shows. At one point he literally lost his voice for a few days and couldn't even speak. After a couple of doctor's appointments, he decided to step down as lead vocalist lest he loose his voice forever.
That being said, it was always intended that once his voice was healed, he would come back, so when choosing a new singer, they deliberately wanted someone who sounded quite different, because eventually, they would have the two singers side by side. Zak's smooth voice with his bluesy Southern drawl not only contrasted sharply to Jon's more screamy metal vocals, but hi more commercially palatable sound (and good looks; the ladies do love Zak) brought the band to their commercial peak with EOT.
Beyond that, though, Jon also wanted to use the time off to branch out into new projects. He stated up Romanov (which evolved into TSO) during this period, and when his voice was strong enough to sing on an album (although still not ready for a full tour) he did the Doctor Butcher albums with Caffery. It was intended that Jon and Caffery would rejoin the band together for the next album around 1995.
Unfortunately, Criss died, and that threw everything out of balance. Jon sank into a deep depression, and was only pulled out of it by working with Paul on the next record. Originally they just wanted to do one song (Alone You Breathe) as a tribute to Criss, but the label would only take a full album. At the time, the rest of the band was so in shock from the death of Criss, they were not yet ready to do a new record. For all intents and purposes, there was no band at the time, just Jon, so he just started recording everything himself. He did everything but the lead vocals (he said they had to "bribe" Zak to come back) and some of the more technical guitar solos (Caffery had been asked, but Caffery was still not yet ready to step into Criss Oliva's shoes) so Alex Skolnik of Testament, who had been a friend of Criss, came in to fill that void, albeit only briefly.
Eventually, Middleton made his way back to the band. Wacholz only did promos and the music video for the title track. Once it became obvious the record wasn;t going to be the success of EOT, he ditched Savatage, and Zak recommended Jeff Plate from his old band Wikked Witch. Skolnik left after the tour, and Pitrellit was brought in as the new guitarist, forming the lineup that made Dead Winter Dead, The Wake of Magellan, and basically became TSO. The rest they say is history...