This settings are meant for HD audio playback at 24bit depth and 96000Hz frequency. So there's no need to change anything default if your audio sources have standard CD quality.
defaults.pcm.!rate_converter "samplerate_best" defaults.pcm.dmix.!rate 96000 defaults.pcm.dmix.!format S24_LE
An exclamation sign causes a previous definition to be overridden. This syntax can be used with any configuration file assignment.
Dmix is enabled as default for soundcards which do not support hardware mixing.
Since Pulseaudio server is not intended to deliver high quality streams for the sake of usability, it is recommended to just not install it.
resample-method = src-sinc-medium-quality default-sample-format = s24le default-sample-rate = 96000
SRC resampling algorithm is very cpu intensive. Setting it to medium quality should work better.
List of linux-friendly audiophile sound cards
Audiotrak Prodigy HD2
Checking actual output parameters
Check the contents of /proc/asound/cardX/pcmYp/subZ/hw_params, where X, Y, and Z are numbers depending on your system.
In order to find this file, execute the following command while playing some sound:
find /proc/asound/ -name hw_params | xargs -I FILE grep -v -l "closed" FILE | grep '/proc/asound/card./pcm.p/sub./hw_params'
Indeed, there exist a hw_params file for each sound input/ouput subsystem whose content is either "closed" if unused, or the actual format of the played sound.
Here is an example of an hw_param contents for a stereo 96kHz / 24bits:
access: MMAP_INTERLEAVED format: S32_LE subformat: STD channels: 2 rate: 96000 (96000/1) period_size: 9000 buffer_size: 288000
More info available in the alsa documentation