Try some new rounds, Dunlop Super Brights, for instance. They say 'super bright' in bigger letters.
Flats are not known for sizzling highs, especially compared to rounds.
Basically i don't know what's my current strings, i bought my first Thunderbird used 2 days ago and i wanna start customize it to my style. But i have a Crews Maniac Sound Uncle HB bass and it has Ernie Ball strings too, although it's nickel wound.
i was thinking in Ken Smith strings too (especially Bass Burner NPS Series), all of them is round wound.