I don't know much about the newer PPI PC series. I know they are discontinued along with the Phantom series. Honestly, the only modern PPI amps I would consider buying new would be the Ion series. They are Korean made and offer decent bang for your buck. But, they are very limited on features and are tiny. More of a cheap daily driver solution really.
Epsilon is the parent company of both PPI and Soundstream. I'd stay away from both brands if you're looking for higher quality amplifiers.
Take a look at NVX or JBL instead of PPI. That brand (along with Soundstream) is dying a slow death.
I doubt there are any internals differences between the PPI, NVX and the Soundstream. Supposedly the Hertz or Nakamichi are slightly better but, that hasn't been confirmed by anyone I know of. You'd be fine with the SS. Same power and output. I just hate the stupid spider on the top! LOL!
I am not impressed with those new Traxx amps PPI has released. They look cheap and I'd stay away. Your Ion 650.1 is a good little amp. The Ion 1000.1 doesn't do rated but, will do 850-900 watts at 1 ohm. However, they aren't very efficient at 1 ohm. But, they will give you about 200 more watts than your 650.1 and they are stable at 1 ohm. Some say the 650.1 will work fine at 1 ohm but, I don't think I'd trust it.