I should add that, per this post, that tool will probably go offline when it comes time to renew the domain. That’s several months away, but after that it’ll be offline.
You can, of course, download the desktop version* and use it locally, or you can pull the source and you update the data files for it (there was never enough data involved to require an actual database, so it’s all just in JSON files) and then rebuild the package, if you want to add other amps, DAC/amp or headphones - since the W4 is not in the current data set.
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*There’s a pre-built, signed and notarized, macOS build there - if you want it for Linux or Windows you’ll need to run an appropriate build yourself.