Ubuntu Studio 20.04 + Ardour 5 + EMU 0404 PCI // Stereo-input for recording not working

Question

I'm absolutely stuck on getting Ubuntu Studio 20.04 + Ardour 5 + EMU 0404 PCI working as it should.

I installed Ubuntu Studio on an old PC of a buddy of mine. He wants to do some recordings and it is perfect to "recycle" the old machine. This PC includes a EMU 0404 PCI.

I have already googled many sites and did many things, such as:

Re-installed Ardour & Ubuntu Studio controls
Re-installed jack
Tried to install driver/firmware for EMU 0404

What is working:

Playing recorded stuff
- Output to stereo-out works
Jack seems to work
Running a Behringer U-Phoria UM2 (USB) Device worked
- Both playback and recording
However, the EMU 0404 (already included in the PC) could not be used

What is not working (and the reason I'm posting this question):

I can not use the EMU0404 stereo-input to record stuff
- No matter what connections in Ardour I make or what additional devices I add in the Ubuntu Studio controls
- I never got it working, that the (directly into the stereo input) plugged in guitar gets routed to the input of Ardour

Due to the fact that everything is there and and works, I think that I don't understand all the dependencies in the whole system:

Ubuntu Studio Controls
QJackCtl
Pulse Audio
Ardour Channels
Buses
Hardware
Settings

I have no clue what to try next, this is the reason why I'm asking for help of you guys. But I'm annoyed that I'm near the finish and only the very last thing ("Getting the input-signal working") is not working.

Below I have tried to post as much information as possible, so maybe one of you can tell me what I am doing wrong!

inxi -Fxxxrz\n\n

System:
  Kernel: 5.4.0-56-lowlatency x86_64 bits: 64 compiler: gcc v: 9.3.0 
  Desktop: Xfce 4.14.2 tk: Gtk 3.24.13 info: xfce4-panel wm: xfwm4 
  dm: LightDM 1.30.0 Distro: Ubuntu 20.04.1 LTS (Focal Fossa) 
Machine:
  Type: Desktop System: Olidata S.p.A. product: N/A v: N/A serial: <filter> 
  Mobo: ASUSTek model: M2NPV-VM v: 1.xx serial: <filter> BIOS: Phoenix 
  v: ASUS M2NPV-VM Revision 5005 date: 06/02/2010 
CPU:
  Topology: Dual Core model: AMD Athlon 64 X2 5600+ bits: 64 type: MCP 
  arch: K8 rev.F+ rev: 3 L2 cache: 2048 KiB 
  flags: lm nx pae sse sse2 sse3 svm bogomips: 11224 
  Speed: 1000 MHz min/max: 1000/2800 MHz Core speeds (MHz): 1: 1000 2: 1000 
Graphics:
  Device-1: NVIDIA G84 [GeForce 8400 GS] driver: nouveau v: kernel 
  bus ID: 01:00.0 chip ID: 10de:0404 
  Display: x11 server: X.Org 1.20.8 driver: modesetting unloaded: fbdev,vesa 
  resolution: 1920x1080~60Hz 
  OpenGL: renderer: NV84 v: 3.3 Mesa 20.0.8 direct render: Yes 
Audio:
  Device-1: NVIDIA MCP51 High Definition Audio vendor: ASUSTeK 
  driver: snd_hda_intel v: kernel bus ID: 00:10.1 chip ID: 10de:026c 
  Device-2: Creative Labs CA0108/CA10300 [Sound Blaster Audigy Series] 
  driver: snd_emu10k1 v: kernel bus ID: 02:08.0 chip ID: 1102:0008 
  Device-3: Thomann SWISSONIC EasyKeys25 type: USB driver: snd-usb-audio 
  bus ID: 2-3:3 chip ID: 07fc:1110 
  Sound Server: ALSA v: k5.4.0-56-lowlatency 
Network:
  Device-1: NVIDIA MCP51 Ethernet vendor: ASUSTeK type: network bridge 
  driver: forcedeth v: kernel port: f200 bus ID: 00:14.0 chip ID: 10de:0269 
  IF: enp0s20 state: up speed: 100 Mbps duplex: full mac: <filter> 
Drives:
  Local Storage: total: 698.64 GiB used: 31.88 GiB (4.6%) 
  ID-1: /dev/sda vendor: Hitachi model: HTS541075A9E680 size: 698.64 GiB 
  speed: 3.0 Gb/s rotation: 5400 rpm serial: <filter> rev: B480 scheme: MBR 
Partition:
  ID-1: / size: 668.19 GiB used: 31.74 GiB (4.8%) fs: ext4 dev: /dev/sda2 
  ID-2: /boot size: 944.6 MiB used: 142.1 MiB (15.0%) fs: ext4 
  dev: /dev/sda1 
  ID-3: swap-1 size: 17.82 GiB used: 0 KiB (0.0%) fs: swap dev: /dev/sda3 
Sensors:
  System Temperatures: cpu: 40.0 C mobo: N/A gpu: nouveau temp: 31 C 
  Fan Speeds (RPM): N/A gpu: nouveau fan: 0

I have already tried to re-install the alsa firmware:

cd ~
wget ftp://ftp.alsa-project.org/pub/firmware/alsa-firmware-1.0.28.tar.bz2
tar xjf alsa-firmware-1.0.28.tar.bz2
cd alsa-firmware-1.0.28
./configure --enable-buildfw
cd emu/
make
sudo make install

Here are as many screenshots as possible:

Ardour 1