Q: alsa audio device reordering: cannot find the slot for index 0 (range 0-1), error: -16 I had no problem for weeks using this configuration to fix the USB card as the primary one (had two, mainboard one and usb): /etc/modprobe.d/alsa-base.conf:
options snd_usb_audio index=0
options snd_hda_intel index=1

Now it suddenly stopped working. This happened at some point after adding a new graphics card (with different drivers) which now adds several "hdmi" audio devices. I think this is related, although initially I thought sound still worked after adding the graphics card.
The problem manifests as such that lsusb lists the device, but not /proc/asound/cards and when plugging in the device I get
[   80.034745] usb 3-3: new high-speed USB device number 7 using xhci_hcd
[   80.163174] usb 3-3: New USB device found, idVendor=1235, idProduct=8200
[   80.163180] usb 3-3: New USB device strings: Mfr=1, Product=3, SerialNumber=0
[   80.163183] usb 3-3: Product: Scarlett 2i4 USB
[   80.163186] usb 3-3: Manufacturer: Focusrite
[   80.165055] snd-usb-audio 3-3:1.0: cannot find the slot for index 0 (range 0-1), error: -16
[   80.165061] usb 3-3: cannot create card instance 0
[   80.165068] snd-usb-audio: probe of 3-3:1.0 failed with error -16
[   80.165301] snd-usb-audio 3-3:1.3: cannot find the slot for index 0 (range 0-1), error: -16
[   80.165305] usb 3-3: cannot create card instance 0
[   80.165309] snd-usb-audio: probe of 3-3:1.3 failed with error -16
[   80.165362] snd-usb-audio 3-3:1.4: cannot find the slot for index 0 (range 0-1), error: -16
[   80.165365] usb 3-3: cannot create card instance 0
[   80.165368] snd-usb-audio: probe of 3-3:1.4 failed with error -16

Now, replacing the above /etc/modprobe.d/alsa-base.conf with
blacklist snd_hda_intel

Fixes the problem, this makes all the sound devices but the usb one which I want to use disappear.
This is not actually what I wanted to do. How do I correctly configure it to give the USB interface priority without removing the others completely?

A: This may or may not help you, but in my case, when I was getting errors like that ("cannot find the slot for index", and "cannot create card instance"), I edited my /etc/modprobe.d/alsa.conf file (differently named in Gentoo than in your system, apparently), changing lines like these:
options snd cards_limit=2
options snd-usb-audio index=2

changing the 2s to 3s.
