The S400 looks like an Amphion bookshelf except with a 5x8 passive in the back.
And the S400 has the tweeter mounted below the mid/woofer, which I rather unlike. But the Buchardt is a newer design so all else being equal, should equate to a better product.
The Amphion speakers are promoted as being very room/placement friendly (no doubt a product of controlled directivity, but that still doesn't explain how bass "friendly" they can be if pushed into a corner).
Unfortunately there aren't many places to audition either in the U.S. Due to direct sales the Buchardt is less expensive to buy.