Ubuntu – the relation between ALSA and PulseAudio sound architecture

Answering the two questions:

You can select the default device in PulseAudio with a GUI like the GNOME volume control, pavucontrol, or from the command line using pacmd set-default-sink.

By default, PulseAudio opens devices for 44.1 kHz or 48 kHz, whichever leads to lower resampling effort (so 96 kHz audio would usually lead to the device being opened at 48 kHz. Also, we open the devices for S16LE by default.

Now, you have two options. If you just want to play a few files at their native format, paplay --passthrough <file> should do the job for you. You can even specify which device to use.

The other option is to globally make PA use 96/24. You can change that in /etc/pulse/default.conf (alternate-sample-rate = 96000, default-sample-format=s24-32le). This will lead to greater CPU usage, though. This is a bit of a silly situation, having to apply global configuration for all devices. We're planning to add per-device configuration at some point.

Note, after this, you should just use mplayer's PulseAudio backend rather than have mplayer talk to ALSA and then ALSA to PulseAudio. You can do that with mplayer -ao pulse ... and if you want to set it up permanently, add ao=pulse to ~/.mplayer/config.

As someone used to "plug and play" audio devices (eg never have to configure anything for sound) your problem "sounds" very complicated to me. However, there are generic problems with equalizers and the accepted answer there might help:

To start the pulseaudio equalizer run:

qpaeq

If this gives you the error

There was an error connecting to pulseaudio,
please make sure you have the pulseaudio dbus module loaded, exiting...

then load these two modules using pactl:

pactl load-module module-equalizer-sink
pactl load-module module-dbus-protocol

and, to make these changes permanent, edit ~/.config/pulse/default.pa (create it if necessary) and add these lines:

load-module module-equalizer-sink
load-module module-dbus-protocol