Here we go again...
I own a Gibby Tbird and have played on the Epi a few times, and I can tell you, its well worth the $400 bucks.
Tbirds are not mud at all, roll off the bridge pup a bit and see what I mean.
Tbirds are not heavy at all, the necks only seem heavy and dive when strapped on because there is no top horn (DUH). If you put a button behind the 12th fret, you wouldnt complain.
Gibson Birds are all Mahogany neck through with ebony fingerboards (very light, mines a '91 and about 7-8lbs). The Epis are mahogany body, bolt on mahogany neck with rosewood fingerboards. And weigh pettym uch the same.
The newer Epi's have nice sounding pups and come with a good factory set up (atleast the ones guitar center gets).
Never listen to the negative things people say about Birds, and Gibson basses in general. If you cnt EQ in a non mud tone from a Tbird, theres something wrong with you.
Oh, Fender Jazz, Fender Smazz, you'll never find a passive jazz bass in the Epi's price range that will have the volume and low end of a Thunderbird, EVER.
I swear, my '91 is the loudest passive bass I've ever played (except for a 2005 a played a few months ago)

.