Bater,

To me it sounds like your mic is just fine, and that the harshness probably comes from your tube preamp. Art's have that characteristic colored sound, and you probably would want something more transparent for your voice. So, I think you need an alternative preamp for your voice, unless the Art has an option for switching off the tubes.

Perhaps you would probably benefit from just having a good soundcard like TC Electronics Desktop Konnekt 6 (just to show it to you: http://www.thomann.de/dk/tc_electronic_desktop_konnekt_6.htm ). It is very transparent, has great preamps for the price, and comes with great A/D converters also (be aware, you need 1394 firewire slots on your computer).

Of course you could also get a compressor or a better preamp (yeah, always suggestions for new gear...) working with the sibliances on the way in, but Sonar should have a plug-in compressor to take care of that (de-esser), doesn't it? But it won't help you if the Art is coloring.

Try to turn down the settings on the Art, and listen at high volume to the top end of your voice. If you like that better, you should bypass the art for your vocals and plug into a good soundcard instead IMO.

If you don't, just try to put a de-esser on your vocal track, and EQ your voice with a lowpass filter to your preferences. If that works you won't need new gear for the moment.

To my ears, what you have sound pretty good, except for the tube coloring from the Art, that is anything but 'smooth'. I would not go with a ribbon mic for vocals in your situation, as you would need to max out the Art to get the required volume from it, resulting in your voice sounding even more like you don't want.

Therefore I would not go with the tube mic, either, as I suspect the tube coloring of your voice is what you describe as 'harshness' and 'sibliance'.