Really funny... Really! PPI is about as old-school as I can remember for a high line of amplifier that disappeared from the mainstream. If ya really want a good amplifier, consider them all, and find a dealer that will let you here the one your after. Never buy an amplifier without know what it can do (even though 10 years ago, I bought my Soundstream 3 months before it was released - but Cleric is right, they have gone through many changes, and might not be the same as I remember them). Rockford Fosgate - Almost always has been a great item. Soundstream - In my mind is/was a great company, but things may have changed (I haven't owned one in 9-10 years that's a long time). Kicker seems to be better than it used to be. Pioneer/Premier is what it has been for the past 10 years - Highly technilogical and innovating and integrating (with it's own products - usually in the Premier line), but (with only a few exceptions) not high powered. PPI - well from my experience, they were the thing to have 15 years ago, but I haven't heard nor seen much of them since then. JL Audio - Awsome high end speakers, awsome high end amplifiers (not much else to write home about). MTX - seems only to get better and better with age. Hifonics - Was the thing to have 15 years ago, and seems also to be the new up-and-comer, as they seem to keep improvong their products - even if it sometimes seems slower than normal. Earthquake - The DR-2 amplifier put them on the map (after trying really hard for years and being very little known) but I haven't heard or seen much else that was groundbreaking from them. MMATS - my only experience with them was a fellow competitor 10 years ago that beat me, and I was impressed - that the competitor managed to fit 20 10inch subs into a Civic - but he still got 6 db's more than my 3 JL 12w6's. And with apparently (by the manufacturers specs) 5500 watts more (total rms - not rated) power than my 1150 watts total rms. Jenson/Craig(although used to be good before my time, and once associated with pioneer)/Boss/Roadmaster/Rockwood/audiovox/Teac/and any other name you can think of that you have never seen outside of walmart/sears/auto-parts-stores/etc, are usually very cheap... You get what you pay for. I paid over $700 for that one Soundstream amplifier 10 years ago. It was worth it. If the same thing (or it's closest competitor) is listed for $100-200 now, it probably isn't worth it.
Getting a used one is a little different though - definately have to have a test drive to make sure it works as well as it is supposed to. I'd pay a nice premium to have back my original (one of the first 50 to come from the factory) Soundstream Reference 1000s. I also had a Reference 700s (which was in less than 3-4 months replaced with the 1000s by the factory) and a Reference 300. I only wish at the time I could have afforded the reference series speakers and subs.
Phoenix Gold - While they do/have made good stuff... They also make some of the cheap stuff too. Beware and only get the stuff they make if you have researched and know what your getting. I couldn't even get the X-max sub the same month they supposedly released it as they claimed they had already stopped production. Two weeks later I had a friend show up with some... They don't seem to treat all of their dealers equal. (granted this was 10 years ago - hopefully management changes) Orion used to be hot - but I've not heard any new innovations. JBL seems to pick n choose when it wants to be top-dog (about once every 7-8 years or so).
Man, there are so many others... I can't even begin.