what amp?

Seriously something with power to get that woofer moving. Alhough I don't think that much of te Mac equipment for my more sensitive set ups I have LaScalas though, The Mac has a synergy with them, also believe it or not older ARC SS power amps if you can ever find one that doesn't fail. Whatever you do don't use a flea powered amp you'll never get the real dynamic power out of the woofer. The midrange horn really dominates the sound when using a low power amp. A nationally known speaker maestro told me that the woofer which is 15" will only move if given the power to. Don't use low power SET no matter what people say sure yuo'll get decent sound but not what they really sound like. If you haven't used a big amp with them lets say 100wpc that's all you need you will never know what these speakers truly sound like. Honest

somewhere in the chain get tubes, you wont be sorry!

Several people have mentioned tube amps as matching well with speakers having horn-loaded drivers.

First I'd recommend that you don't use the sound card. Check out the Squeeqebox2 or 3 model by Slim Devices. Plenty of posts here about pluses of that.
Then, regarding the amp selection, you'd need to mention your budget first, and what kind of music you listen to, your room, etc, or you're going to be getting recommendations all over the board.
Just my suggestion.
Good luck and good listening!