Why am I experiencing listener's fatigue?

Strange you have listening fatigue with this set up. I don't see any components that stick out as a problem, although I am not all that familiar with the B&W speakers.

The Cardas wire is warm and low distortion, the Sony 777 Is supposed to be very good and I know for certain the Manley is good.

Given no other info, you might try a tube DAC with your Sony. I am trying the new Scott Nixon DAC and its tons smoother and more organic sounding than the analog output jacks from my Sony ES.

I guess you've tried the system with the Monster power conditioner removed?

I don't like that conditioner, but it can be a benefit if your power is in bad enough shape.

Could be set up problems. Do you have first reflection points under control? What have you done with toe in? Are you listening on speaker axis? You don't mention room conditions, size or location of speakers and listener position at all. Would listening with the speaker grills on help tame the highs?

You might also look at what you've got supporting your Manley. Possibly resonances affecting your tube integrated?Tubes tend to amplify resonances in addition to the signal. You might want to consider how much toe-in your B&W 805s have. Depending on your room, pointing the tweeter directly at the listening position might be too much, as Newbee alluded to. Good luck.

I think Cardas wire is boring to listen to and might lead to fatigue. Also I am not the biggest fan of the B&W sound, I think they can be dry and wooden, not the most expressive speaker. How are the tubes in the Manley? I really love the Stingray, it is a great piece...

I agree with Phil. IMO B&W's can be dry and border on bright. I did listen to the new diamond series by B&W and was very impressed. It did not sound like B&W, in a good way. Room treatments can help quite a bit. You might try a vinyl setup :). If it were me I would look at replacing the speakers, just my opinion. Good luck.