Audigy FX:
Uses a MAX97220 headphone driver (on front and back outputs), like their E1. That gets 2 Vrms from a 3.3 V supply, thanks to an integrated charge pump. 22 ohm output impedance, plenty OK for 300 ohm cans. Distortion across frequency is quite flat at around -80 dB 2nd and not too impressed by output loading, I've seen much worse from this kind of IC. Performance degrades in the bass though. Always sounded fine to me with HD580s.
The audio codec is a Realtek ALC898, attached to a PCIe HDA controller. DAC is fine, ADC basically also is but digital filter periodic passband ripple isn't exactly measurement grade @+/-0.025 dB or so (ALC889 was better). Also, avoid recording at 44.1 and multiples, performance degrades a lot (PLL jitter?). Line in about 22 kOhm input impedance @ ~2 Vrms. Mic in is better than average for Realtek but bias voltage noise still obliterates (not very low) input amplifier noise.
Not sure whether that's fixed yet, but the Win7 driver at least has a somewhat annoying bug that lets the volume settings on FL/FR and the other channels go out of sync when alternating between front headphone and rear output and adjusting volume while headphones are in. The effects that are enabled by default are the first thing I turned off.
The Xonar SE seems like Asus' more modern answer to that particular card. Why modern soundcards always seem to be using USB audio controllers beats me, but whatever. No extra headphone driver is provided here, but from what I've seen the Realtek S1220A (ALC1220) should have half-decent headphone driving built-in, and can output 2 Vrms. From what I've seen the newer codec seems to be well-behaved even at 44.1. I may actually still give this one a shot myself. (I've been meaning to build a new PC for a while now and originally intended to use a Xonar D1 or D2, but from the looks of it the few Ryzen boards with PCI slots would still be problematic with these. Bummer.) SNR specs are close to the D1/DX, not sure about actual performance. Distortion should be substantially worse though. Drivers just might be somewhat less of a mess. (Looking at Amazon reviews, well, I'm not so sure.)