What is the best way to tame a slightly bright speaker?

Vandersteen CS2 Signatures use ear bleeding tweeters, but they run the audio signal through 3 capacitors in series. I am not sure that I would recommend doing that, but adding a solen cap or padding the signal with mills non-inductive wire wound 1 ohm resistors may do the trick. Cables may also help, but when I was having issues, I finally, out of desperation as I never really bought into AC conditioners bought a hospital grade isolation transformer and that cured my problem. Also, for under $100 for SE, and I suspect under $150 for balanced ICs you can get ICs made with Dualund's improved version of Western Electric wires, and I'd be pretty shocked if they don't sound better than your Analysis Plus ICs, but it certainly won't break the bank to find out.

Room treatment for sure, power conditioners and cables will not tame that speaker, if they do they are affecting or colouring the signal which they all claim not to do. Your speakers are a bright speaker, I’m very familiar with them. You can spend 000’s on cables, power and upstream components but the fact of the matter is in your room with those speakers interacting with the room and everything located within it, they are intrinsically a very open, “brighter speaker”. Also try repositioning them, less toe etc. That may work. But, and I say but, your best first option as mentioned above is room treatment, it is an inexpensive trial that I learned about years ago and what a difference it makes. More difference than any cable and especially and power conditioner!!

I think that either the speakers or the amplifiers may be bright sounding, however I wouldn't start by replacing them. An overly simple solution might be to fit a small piece of foam over the tweeter, one where you could add a piece of cloth over it to increase it's ability to reduce the amplification. 

Not pretty but not expensive and not permanent and adjustable to your liking. 

I feel the best bet to tame a slight bright issue would be to try smoother sounding cables. There are so many to choose from. You can try interconnects or speaker cables. For me, I find anti cables sound smoother and not as bright as many cables out there. 

Install a dimmer switch.