HELP Why no bass? Why bright?

...or face the other possible that can happen: you can simply adopt to that brightness or tightness and say OK you were guys right, after 600 hours the system had opened and is no more bright!

Regardless of the tweeters, etc, I assume there would still be no bass.

Why are you running another integrated with a power amp?
Have you considered getting a real preamp (preferrably, but not necessarily one with tubes)? A good preamp would probably solve most of your issues.

Are you bi-amping? Many times this is why companies like Arcam have a matching amp for an integrated. Running a new integrated with a power amp (and also not bi-amping) is not really that much of an upgrade, mainly from the use of the integrated as your preamp, matching power amp or not.
Bi-amping with an integrated usually assumes the matching power amp is the same watt output as the integrated's amp.

I also agree with the break-in period, not only for the amplifier, but the speakers as well. Those kevlar drivers have a long break in period that your old amp may have been masking.

B&W speakers are not known for deep bass(as are most Brit speakers)...they are designed for classical in mind afterall...and the same could be said for Brit amps...they can be a bit lean for some applications...most Brit equipment is "all about the midrange"...still...there should be some semblance of bass present...even within their limitations...B&Ws dont strike me as overly "thin" sounding...they are considerably brighter sounding than Spendors,Proacs,Castles,etc...but these are "rolled off" in the highs...so the comparison is somewhat exaggerated....

ALso...if your B&Ws are on loan...and you detest brightness as you stated...going with a soft dome tweeter ala Spendors,Castles,Proacs,etc might be the ticket...although this will not solve your bass issue...if you really crave more bass...you will have to add a sub to just about any monitors to achieve it...or go with a floorstander...which is another set of trade offs all together....

ALso..to address your cable concerns...analog guys love Kimber cable for its warmth...and I have heard from fellow posters DH labs cable can be bright with the wrong equipment....I hope this helps...just for the record...B&W speakers seem to be getting "brighter" with every new generation....