You should probably determine if this is a hardware problem that full-on brightness is not enough, or
a software problem of your not liking the incandescent light simulation.
There is a program that will turn the LEDs full-on for testing purposes. It gets installed as pidp8i-test.
On your pidp8i, you'll need to stop the regular pidp8i system to run it:
pidp8i stop
Then you can run pidp8i-test, and it will step through lighting up the LEDs.
If it's a hardware problem, you could experiment with lower values of the current-limiting resistors. But be careful not to blow out your LEDs.
If it's a software problem with the incandescent light simulation, we've been playing with that rather a lot in recent months. Perhaps taking a newer version of the software will give you lighting you like better.
-Bill