Most stations using extended hybrid mode are using the “MP3” version which has nothing to do with the compression mode of the same name.
I don’t have the white paper in front of me but that factors out to 64k on the HD1, 32 on the HD2, and generally 28k on the HD3 with the rest used for data.
KUOW in Seattle operates this way.
For music using the MP1 mode for a full 98k on a single HD stream is the best (obviously) for audio quality. in that mode if you add an HD 2 you necessarily have to cut the quality level of the HD1.
Depending on the quality of the audio processing and music files being played this will be noticeable. At these lower bitrates any decrease in quality will be magnified.
Stacking compression algorithms will also result in audible degradation. Compression algorithms exist in satellite feeds, MP3 music files, STL systems, and some Audio over IP studio/control room systems.
A good example of a minimally compressed system would be KEXP. the only compression in their on air stream is the occasional Apple “lossless” coded song file. or when they are on a live remote. they have a single HD1 at 98k.
I don’t have any direct knowledge of how Eugene stations are configuring their HD but hopefully that helps with the background info referenced above.
Personally I notice Seattle/Portland HD stations of most bit rates sound pretty clean, going over to Spokane they mostly sound full of artifacts and to me after hearing what HD radio can sound like, unlistenable. OPB HD2 could sound better and I have no knowledge of why it doesn’t.
As for Eugene I’ve only heard KZEL and KLCC in HD which both sounded fine.