i finally resolved the annoying issue.
mc21 worked perfectly on direct sound output. i got a hint that wasapi output settings should be looked over.
so i tested changing wasapi options and found out the solution.
i should have changed wasapi exclusive event mode into push mode by putting a check on 'disable event style' on options > audio > device settings.
but, i'm still wondering why aimp4 worked well regardless of event/push of wasapi exclusive.
well I'm glad you sorted it ... but I'm pretty confused. So you are now using Direct Sound instead of WASAPI?... if so you are no longer by-passing the windows mixer.
For that DAC I'd set DSP Studio Output forma to no change up to 96khz (or 192k using certain connections, I think); over those limits set to downsample to either 96 or 192 depending). If you don't have high rez files this of course is not necessary. You can set the output as Matt has tested, but normally you wouldn't want to (IMHO)
I believe their native driver is an ASIO driver ... normally you would want to use their driver, before WASAPI, and certainly before Direct Sound (unless you have other conflicts like with a DAWS?) - I'd verify that the appropriate playback device is chosen in Windows. If using WASAPI with a reasonably modern rig, normally you would want the default event style.
Just googled their support as I don't know this particular version of SB ... but a new driver dated 16 jan was recently released. If you are running Win10 especially you would want this (could be causing the problem); even if you are not running Win10 I'd make sure your driver is up to date
anyways, again good that you found your solution, but you might want to check that your config is optimum for audio on that dac. I'm guessing that is really the root of the problem