Poor play/recording quality with FocusRite Scarlett 2i2

I'm experiencing poor play/recording quality with an old iMac (see below) on which I recently installed High Sierra. Here is a recording. Another example would be when playing a youtube video (in Safari) with output to the 2i2 which results in poor quality as well, then switching output to speakers and it is good.

I have a FocusRite Scarlett 2i2 (3rd Generation) and it sounds fine just playing through it (in and out) since then it is not using the mac audio drivers. The recording is from Audacity, but it sounds exactly like this from GarageBand. I tried clearing PRAM (cmd+option+P+R) on start, but it made no difference.

I have tried deleting your Mac audio preferences files in order to refresh them to see if this resolves the issue. See here: - I'm having issues with my interface on a Mac, how do I Delete my Audio Preferences on macOS?

The 2i2 is compatible with High Sierra.

I've tried both FocusRite Control 3.6.0 for macOS 10.12.6 or newer and 3.7.4, with no difference.

The usb is connected directly, i.e. not through a hub, and I have tried more than one port.

I've also connected the 2i2 to a Win 10 machine and all works well.

What else can I do?