Hi, Neville:
I'm not using the onboard phono stage, nor have I heard any feedback from anyone who is using it, so I'm afraid I can't help on that front. I've always felt that, if you're looking for absolute performance in analog playback, the best way to go is a premium outboard phono stage--but I'm sure that, if the rest of the Concerto is any indication, the built-in phono stage is pretty nice.
As far as comparing the Concerto to the DarTZeel, it's like comparing apples and oranges. The Dart is a dedicated amplifier, not an integrated. It also costs A LOT more--18k to be exact! That said, if you have the cash and have any interest in staying with separates, the Dart should be at the very top of your list of amplifiers--provided that your speakers aren't an insanely tough load.
Unlike most integrateds, I don't think you're giving up a lot versus separates with the Concerto. I've heard a number of people say that it compares very favorably with the Rowland separates--which is saying something. You might give up a little power, refinement, and resolution, but I highly doubt that it's going to be a night-and-day difference. If you're set on downsizing, I can't think of a better integrated to do it with. You might also want to audition the Musical Fidelity KW500--it has twice the power, but I don't think is as quiet or revealing as the Rowland. It's definitely a very worthy contender, though.
I sincerely hope this helps in your quest. Maybe some other Audiogoners have other suggestions that I may have missed, but I think the Concerto is in the top tier of integrateds and deserving of a serious audition.
All the best,
Andy