Aftermarket fuse to tame a bright system?

For now, do nothing.  You need to get accustomed to the new sound.  A lot of the break-in process, I suspect, is the listener breaking in, not the gear. Also, try not to move the new cables at all; let them rest in their current position so that static charges in the insulation stabilize and become evenly distributed. After that, you might need to tune the system if it still doesn't please you.  Any new addition is not something that you can drop in in place of something else; you have to make adjustments. 

The first adjustment would be speaker placement or listening chair placement because these are free changes.  Sometimes it is a shockingly small movement of the speaker will do it.  In particular, change the toe-in of the speaker to primarily alter the energy delivered by the tweeter.  Moving the speaker may allow you to find a placement with more bass reinforcement--more bass means the relative balance has shifted downward so the perceived brightness is reduced.  

I doubt that a quick and easy hardware solution, such as fuse swap will really do the trick if there has been a substantial change.  Perhaps the best hardware solution might be to put back one of the interconnects, or the speaker wire, s that the entire system is not cabled with the Audioquest stuff.

Speaker placement/toe-in changes that will make a substantial difference can be quite small and may even help with other aspects of performance.  This is the cheapest, and probably the best fix; if it doesn't work out, just undo the "fix."  I don't believe one needs to use the same cabling throughout, so swapping in one or two other cables to see if that is the fix is a good approach too; it will cost you nothing if you kept the old cables.  

The last thing would be spending money (although that can be fun anyway).  Power cord changes probably won't do the trick--good power cords tend to help with dynamics, clarity and sometimes soundstaging, but rarely fix brightness--but, then again, they are not likely to make the sound worse, so that it not so much a gamble.  

Are you doubting the magical properties of $600 fuses?  Even the ones whose conductor is made from fairy pubic hair?

I know you are trying to compensate for a problem that only arose when you changed cables to the silver Audioquests.  That brightness (meaning comparatively lower levels of the frequencies below those that now sound bright) might also account for the clarity and increased dynamics so that taming the brightness may mean losing what has been gained.  If your older cables were somehow hiding a defect in another part of your system, then changing out the problem component could cure the problem.  It might not be easy finding the problem component(s). 

Compensating with cheap additions, like the fuse, may alter the balance, but I suspect that that effect would be quite subtle.  If the fuse was choking power availability to the amp such that it would grossly affect tonal balance, the amp would have to be quite inadequately designed.  That effect would also only be evident at high output levels where the amp is more likely to be starved.  This doesn't really explain why the problem was only revealed by the new cabling unless the speaker cables are forcing the amp to work harder.

You bought new cables to change the sound of your system, which they apparently accomplished.  Any change has a good chance of being negative or having some aspect of the change being negative.  Any substantial change means also tuning the overall system to balance various tradeoffs that come with a change.  That means doing the things, discussed above, like adjusting speaker/listening chair position.  I hope a simple fix, including the fuse experiment works; otherwise, you might just have to abandon the new cables.  Silver cabling can sound harsh or bright in some systems, and the really good silver cables that largely avoid this problem that I know of are very pricey (e.g., Audio Note silver cables).