Interconnects and speaker cables can yield minor changes (usually very minor) in the perceived frequency balance of a system, but the single factor that most often contributes to lack of highs is the listening room and its acoustic characteristics.
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You did not describe how your room is furnished, but if you want to add more high frequency emphasis to the overall sound balance, you might try adding some harder or more reflective surfaces to the walls or floor of your room. Alternatively, if your room has a lot of heavy furniture or drapes, you might try removing some of it.
There have been a number of threads on this topic over the past several years, so you might find it helpful to peruse the A-gon archives.
