Thanks to everyone who wrote in so far. I read somewhere that it can take up to 100 hours for a new component to fully break in? I can't imagine myself that the sound will change that much anymore. I played it for 3 hours the first day and about 9 hours yesterday. I'll give it a further good run in this weekend.
I'm getting a sharp sound with sibilance, the T and S sounds with voices are very shrill. Somwhere in the upper midrange and lower treble region is the part that is bothering me. I guess I can be pretty sensitive to this. A componet needs to be smooth in this area for me.
I owned a used Arcam Diva A70 for a very short time about a year ago and I don't remember it being as bright as the A18 is. I paired it with my Proac's and the sound was smooth with very few traces of harsh sibilance.
The one varible that I also changed in my system is my CD player. I still have my old one and haven't sold it yet. It is a Cambridge Audio 840c. I think the C.A. might be a bit better player than the Arcam is? Not sure. I got the Arcam CD17 because I wanted it to match the Arcam A18 and I heard that it's good. But maybe the CD17 is the culprit? I'll have to get the 840c back out of its box and make a comparison.
The cables I'm using are known for being smooth and for taking off any slight edge in the sound. That's why I'm using them. Perhaps I can still try some other brands and types. I never heard the PMC's in person before. That might be somehting to look into as well. My Proac's are getting on in age, 12 years old now.
One other note, my last amplifier was a Creek 4330. It broke down on me a few weeks ago so that's why I bought the Arcam duo. Now that is a great amp! Smooth and sounds powerful. They don't really make them like that anymore. Passive pre amp and mosfet output stage. Never had any brightness problem with it. The Arcam does beat it in detail retrival though and with smooth recordings can sound quite magical.