I'm not hooking foobar to G61 I'm hooking it to a DAC/Amp via WASAPI or ASIO.

I've used the Explorer 2 , iFi iDSD BL and Chord Mojo and they all sounded fine and ran BitBerfect. I could tell they all ran bitperfect because the Mojo and iDSD could natively decode DSD and the Explorer 2 was Natively decoding MQA.

If your running an HTPC to HDMI out to HD621 that might involve a whole other set of expertise to set up correct.

Unless you have DTS and Dolby MUSIC files in your library I would not bother with Foobar2000. I'd use JRiver (if only needed DSD). If all PCM 2 channel FLAC I'd use MusicBee or MediaMonkey. And even if I didn't have DTS or Dolby in my music library I would have one DTS embedded FLAC file to confirm my setup is bitperfect to the preamp. Player doesn't need to know anything to play that. This confirms nothing is nucking with the data stream.

If you are watching movies from PC I'm not sure what the best solution is.

But I would confirm that you can play a Dolby Digital encoded movie and see your Preamp say AC3.2.1 to confirm you have a bitperfect setup. Not sure the how feasible lossless codecs are on a PC are (which primarily what an HD621 is for).

Once setup correctly I've never heard one app sound better than another nor one connection type sound better than another including optical out of a Mac Book.

- Fiddler (IMHO)

If you think listening to music is fun, try learning an instrument.
HT: G61RSL, 4x5K, 5.5KHC, 2xSC6000, Transporter, HD621, Philips BDP7501 4K, Tivo, 65" LG OLED 4K C6
Computer: Foobar2000, 5K, Mojo, HiFiMan HE1000 V2