Question about Creative Sound Blaster ZxR

Well, if you clicked on the "specifications" link here at Digitec, you would get to a website. And there you can read the following: Headphone amplifier up to 600 ohms. Everything clear?

More impedance (with unit Ohm, describes frequency-dependent resistance for alternating current) means less current flows, which tends to mean quieter (or simply "only" higher-quality technology), but should not be a problem for a sound card.
Rather the opposite: too little impedance leads to too much current. The amplifier chips on the sound card cannot supply this large current and "saturate" with the consequences of bad sound up to the destruction of the amplifier (= sound chip / sound card, or its fuse).
(This is theoretical knowledge, in practice I have always connected everything everywhere without problems, but I have never tried the case of low impedance speakers to high impedance amplifiers).