Can someone please explain to me how Ohms and Hertz affect the quality of a headphone? I've recently got into the hi-fi world, I am a completely confused as how this all works. I recently bought a pair of Harmon/Kardon CL's and a Creative Sound Blaster ZX sound card. With it comes a built in DAC and 600 Ohm impedance built in amp, I thought this would power my HK and give it a little more oomph since they tend to run a little low (My traveling headphone since they're compact, closed-backed, and the cups swivel).
Now, I have had plans to buy a cheap FiiO E6/E03k (Upgrading to the better E17/E09K when I bought my Beyerdynamic/AKG's) Amp/DAC combo on amazon for my sennheiser HD449's and Fisher Audio FA-004. The HK's sound really bad with the built in amp of my sound card, like I am underwater. The Sennheiser HD449's sounds a lot better with the amp, the bass is much more responsive. So I know it works. What combo for the HK would work best for a amp/DAC combo, I was thinking the E6/E03K combo from FiiO but I am afraid I will not purchase the right items. I just want a little more power out of my HK's. Here the sound card specs below-