Just to jump in as defender of the Xonars. I have never had an STX II, but I am using a Xonar D2X as only player for my music, which is flac files, stored on an 1tb ssd. As the D2X is a 7.1 card, I have got to reroute the side and back channels, which I'm doing with EQ Apo. So I have 6 channels stereo, 1 channel for the subwoofer, and 1 channel is not used atm. The crossovers are made with rePhase, and integrated in the EQ Apo setup via the convolver.
As crossover for the subwoofer, I'm using the "flexbass"-feature of the Xonar driver, which takes the sub out at 80hz with 36db/oct. . I made several attempts to let this task also be done by EQ Apo, but I always find myself back using the Asus-tool. The woofer Chassis is a 12" Mivoc AW 3000, which is not the perfect sw, because as it was constructed, it was defined, as a woofer, which can also be used as a sub. In simulation, a not very expensive GRS 8" HE lets the Mivoc bite the dust in -3db frequency, as well as in amplitude, and that not by the width of a hair. So this will be my next construction site (I'm fine with the Mivoc, it's driven by the EQ Apo settings and a 250W class-D amp to where no Mivoc was meant to go, but I know I could have a lot more possibilities in reproducing lowest bass, by just using two of the 8" GRS instead of the 12" AW 3000.
Stereo goes to 2.5-way floorstanders with 2x 5.25" midbass from sw up to 600hz, then taken out with 48db/oct. Another 5.25" goes from sw up to 1900hz, where it hits in a brickwall, from where the ringradiator tweeter takes care of the leftover frequencies. These four chassis are powered with 4 LM3886 Chipamps.
Back to the start: all this is driven by an Asus Xonar D2X. I'm not satisfied with this situation, because I'm never satisfied, but I should be.
As long as I use this soundcard (and I know it's a less than sub optimal task trying to reproduce HiFi in the environment which the inner of a pc case is) I had and have none of the problems mentioned above. My speakers got some hiss, but to hear it I have to put my ear less than 1"/2.5cm to the speaker.
So, it's not the material, it's not impossible, even for an amateur like me, to get some serious sound out of a pc.
I've found the linked YT video just a week or so ago. I've no possibility to do the measurements the guy in the vid does just by the lack of equipment, but the results show the same, my ears tell me.
Focusrite Scarlett vs Sound Blaster AE-9 vs Xonar D2X