Since you like Thunderbird-esque basses...
Jackson has the Kelly Bird basses that are a riff on Thunderbirds and are within your $400- $600 price range, but I have no experience with Jacksons so I cannot tell you if they're any good.
Kelly Bird | Jackson
I would have recommended a Schecter Ultra Bass or basses in ESP/LTD's Phoenix, Stream, and perhaps GB lines, but they're far beyond the $400- $600 price range. I think Schecter and ESP/LTD made solid basses.
Ultra Bass
Products - Phoenix Series Basses - The ESP Guitar Company
Products - Stream Series Basses - The ESP Guitar Company
Products - GB Series Basses - The ESP Guitar Company
When it comes to body shapes, I tend to prefer something akin to the sleeker Ibanez Soundgear shape rather than the Fender shape (which doesn't do it for me). Go figure, my most "comfortable" bass is my F-series LTD. Regardless, Ibanez's Soundgear line has some solid basses in the $400- $600 range:
SR | PRODUCTS | Ibanez guitars