So, I spoke with ATI Tech Support, and he said the Signature Series is their best sounding amp.
I also talked with a really awesome guy at a big on-line ATI e-tailer and all I got out was. "Hi, I am trying to decide which ATI amp to go with", and before I finished my sentence, he said, "Signature Series". Like 3 times.
Then he left me finish...LOL and then proceeded to explain why. He also told me some inside stuff about ATI among other things. So, I've made my decision on which amp.
I've had a 1505 for 18 years and it has never given me a problem in all that time, so I'm excited to see what they have done since then.