I don't know about your weakest link, but it looks like you've already received some good feedback.
I have the Kiseki Blue N.S. and The Hana SL. The Hana is a great cartridge. It's biggest strength and weakness (IMHO) is that it's incredibly neutral. Strength, because it doesn't add or take anything away from the music, weakness, because other cartridges may be a little more lively, colorful, etc. I definitely find the Kiseki Blue to have a bit more in the way of detail and dynamics and enjoy the sound signature. I haven't heard the Purple Heart, but would expect it to be more of what the Blue offers.
I bought mine from seller kron here on Audiogon. The price was very attractive. It looks like he only has the Blackheart listed at the moment. You might try dropping him a note and see if he can get the Purpleheart if you're looking for a good deal. Keep in mind that it would be a grey market cartridge, with no US warranty support. There have been reports of quality issues with some of the Kiseki cartridges.
kron has a 100% seller rating and my experience with him was excellent and I've had no issue with my Blue.
You have a nice table and phono stage (I have a VTPH-2A also) that should respond well to a cartridge upgade.
Some other things to consider are different speakers, room treatment, or a couple more subwoofers. I've only heard the B&W's briefly (not sure if they were the same model), but you're not the first or only person to characterize their sound as bright.
If you want to get rid of the brightness, different speakers and room treatment are the places to start. Room treatments don't have to be ugly, there are some where you can have photos or art printed on them.
