Lanx0003,
I do own the Buchardt S400 MKIIs, and LOVE them. You are correct their sound characteristics are gorgeous except for deep bass. The previous model of S400 had an artificial "bump"t in the bass, that the designers removed for the MKIIs for a more accurate sound.
Your power is not the issue, I run them with a Denafrips Thallo amp rated at 120W into 8 ohm, and 220W into 4 ohms with no issue. The passive radiator does not add any difficulty for the amp.
The Buchardts with their passive radiator MUST be placed nearer to the wall behind them for proper bass response, and not pulled out into the room. I currently have mine 14" out from the wall, but I'll sometimes move them to 11" when listening to a lot of rock music. I did find that the soundstage started to shrink as I got under about 9".
The bass is quick, accurate to a point, and thoroughly enjoyable. I will however add a sub or two at some point, was thinking REL T/5X or T/7X