The main difference is the addition of the raw mode in channel 2. Also the series 2 has the ability to take EL34's or 6L6's with a bias switch in the back
I found the raw mode a very worthwhile upgrade. I believe there was also an improvement in the fx loop, or perhaps it was an improvement in the channel switching (eliminated a delay?). I had a Series II and really liked it, but opted for the Mark series instead. But I could have lived happily ever after with the ROV!.