Hi Toffeng,
I have heard the Aesthetix Io and an older version of the Rhea as well as various Naim phonostages. The Aesthetix Io, in its various iterations, are very nice sounding phonostages, though they can be a bit noisy, particularly if asked to work with very low output cartridges. The Rhea I heard (in a dealer's system I was very familiar with) was a quite different sounding unit. To me, in the particular system I heard it in, it sounded a touch brittle and more like solid state electronics than tube.
I've heard both the cheaper and the top of the line Naim phonostages in a friends system. I don't like the lower end phonostage because it has a somewhat "dry" sound (it is hard for me to describe the sound, it is more a personal response to certain solid state gear), and has an edginess to the attack of the note that makes everything sound a touch mechanical and artificial. But, the top of the line phonostage is an entirely different matter. It has far less of the kind of unpleasant qualities of some solid state, delivered all the harmonics of complex music correctly, and delivered the kind of dynamics and punch that Naim fans like while minimizing the "mechanical" qualities of the Naim sound. I heard it attached to one of the better Naim power supplies, so, in that configuration, it is quite an expensive phonostage.
Among the not so high priced cartridges, I have heard, and liked, the Transfiguration Phoenix and the Ortofon PW. Both are not as warm and rich sounding as the Koetsu cartridges, but, I suspect they will be a nice match for your system.
