According to reviews the phono stage in the Marantz is very nice. It's hard to to say whether you could improve on it at your budget. What you will probably get with a separate phono stage is more options for loading and gain settings.
The Nova phonomena is a very good phono stage. I have two of them. They're very quiet and have a lot of different loading options and gain settings.
Having said that, I recently picked up a Parks Puffin and like it more than the Nova phonomena. They're $400 new. Very quiet, great soundstage, and tons of adjustments are available. I don't think you'd have much trouble getting most of your money back if you don't like it.
The Nova phonomena is a very good phono stage. I have two of them. They're very quiet and have a lot of different loading options and gain settings.
Having said that, I recently picked up a Parks Puffin and like it more than the Nova phonomena. They're $400 new. Very quiet, great soundstage, and tons of adjustments are available. I don't think you'd have much trouble getting most of your money back if you don't like it.