Recommend me a new PHONO stage.

A used Graham Slee Reflex Era Gold for just a tad over your 500. Awesome phono stage that many sell before they are fully burned in. They take a good 500 hours of constant power to get really going and should be left powered on always. There are others but this one I am familiar enough with to say it. Cheers

Lehmann SE fits that budget, lottsa grunt for your 103 and no real negatives.

NAD PP2 or PP3

Primare R20. Go to their website and research.
There's one here up for sale now. I have nothing to do with the sale or the person.

I read about these suggestions and also about PS Audio phono stage. None of them "won any awards" in sound department.
My fear is that all I would get for extra 300-500 dollars is more features (switches), but pretty much the same sound quality as my basic Rotel?
Aren't there giant killers in this price category?

I haven't heard it but today I asked the same question at my local dealer (whom I trust) and they suggested a Moon phono pre-amp which was in that price ballpark.

I had the Rotel and replaced it with a Musical Surroundings Phonomena and that was an improvement. There was less surface noise, deeper bass, and an overall increase in fullness or body of instruments and voice. The Phonomena has received positive reviews and can be purchased used in the 300.00 range or the new revision 2 for 600.00. You can also change gain 40-60db, capacitance, and cartridge loading resistance.