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).
