I previously created a post with numerous tips, but this time I want to focus on one tip in particular because it's about a very common issue I notice with digital orchestras: 10k+ buildup

Nearly all orchestral libraries have a relatively dry sound out of the box. I mean relative to how the instrument would sound in a concert hall, not totally dry with no reverb. I believe the developers do this so the libraries are more immediately flexible for a variety of styles and give the user more freedom to shape the reverb. This relatively dry sound typically comes from the use of close/spot mics.

These close mic positions contain a lot more detail than more distant mic positions, which means a lot more high end is captured. When you stack many close mic'd instruments, it can create a noticeable 10k buildup, and the individual instruments will lose some definition, and the overall effect is unpleasant.

One solution is to reduce the use of close mics and get most of your reverb and tone through the mid/distant mic positions. But If you want to keep the close mics so you can create your own simulated space, then you can simply cut some of the highs of your instruments to control the 10k area; you don't need to cut 10k on every instrument - you'll usually hear a significant improvement in clarity from making cuts on only a few instruments.

Be careful when cutting 10k because you can easily get a "cheap mp3" sound if you go too far. A simple -1 to -3db bell cut can go a long way.