You've been quite helpful in aswering my queries in the past, so I will try my best to pay you back the favor.
First of all, to answer your first question, I think it is best if the cables used throughout the system are homogenous, and, this includes all the cables used within the hardware and speakers. This is all theoretical, but I believe the best sound quality is achieved when the sonic characteristcs are equivocal from front end to the transducers, and the best way to achieve this is by using all the same cables because this method will have less added sonic characteristics.
If you believe in this theory and want to apply it, ask the manufacturers of your components what cables they use in their system, and get a bunch to audition. I know Blue Circle uses Cardas inside their components, so give them a try. Personally, I would like you to sample the Bear and Virtual stuff because I am curious to hear how the sound from your perspective.
In my own system, I use AQ Emeralds throughout, and they sound great. But, like yourself, I can't help but to want to audition more cables. I've auditioned maybe 5 other cables, but since I don't have 2 pairs of them, I am unable to run the sames ones throughout the system. But when I first plug in the "new" cable, generally I get either more of something, but then I get tired of it and plug my emerald back in. I conclude the added something to be coloration, and using the same cables sounded most pure. Also, my speaker cables are AQ Argents, so all my cables are from the same family.
Secondly, you mentioned your system sounded bright before the upgrades, are the brightness cured by now? If it still is bright, judging from your system, I would say the culprit is your sub. NHT are the dryest sub I've heard. I also tried to find some information on the Proacs to find out how efficient your tab 50s are. I've never auditioned the BC22, but they are only 30lbs, so I question how good they are at driving speakers with higher impedance. If they don't have enough juice, your speakers would sound bright when playing when your playing them loud.
Anyhow, that's my 2 cents worth,