Here is some info I received from Oleg at Coppertino. He is a very nice guy who spends time replying to my emails etc. I hope he will not mind me publishing the excerpt from an email. as it will be of interest to some here.
Your question - does VOX introduce EFX/EQ. The answer is it does, but only if You enable it. Otherwise than this, it tries to keep audio path as clean and processing - free as possible. There's no hidden EQ or enhancing, we just decode file to 32-bit floating - point data and send it to system's device. If re-sampling required - we do it very gently, using 32-point sinc interpolation, and that may be avoided by Sample Rate Sync option. That'd be bit-perfect playback up to 24 bits.
When data are sent to device which does not support 32-bit float sample format internally, these data will have to be converted to a supported sample format (typically, 24 -bit integer), and that's where bad things may come into play, however, that's only my assumption, for I am unable to hear the difference. And I am a sound engineer with 12 years of experience in live sound/studio.
So as for technical information - the system is playing audio for us. Files are decoded by their native decoders and Core Audio (Mac OS X audio engine) is used for playback. In case of mp3, m4a, ALAC, AAC, WAV, AIFF, CA formats, files are decoded also by Core Audio. Currently we use BASS audio library to help manage data flow and control between player and Core Audio. BASS is known to be fast, clean and byte - transparent audio library. BASS handles file reading and decoding and sends it to Core Audio. Using BASS we are able to control where the processing takes place and how helpful / harmful it is.
Typically those good guys who blame the sound are using players that use so called "integer mode" which allows to bypass almost all links of system audio chain, as You correctly wrote, to decode file and send it straight to the device without conversion involved, effectively bypassing most of the goodies of Core Audio. But integer mode inherently excludes all of this stuff our users wanted us to add: effects, eq, crossfade / smooth fades, software volume control. IM is for expensive DACs, so usual listeners won't really benefit from it. I will tell You more - built-in soundcards in today's macs support float 32-bit mode natively so may even sound better than some DACs that mostly do not support it.
Integer mode also requires to load a file fully into memory before playing, to minimise the RF and power voltage impact of a working hard disc. That's also a problem with large files and small memory footprints - I would keep reading file from HD even if some virtual (unnoticed to me) RF of power pulses are having place. That' good enough for me but might not be enough for You. But I will tell You more - one day they will tell that reading from memory also makes voltage fluctuate, we need to read files from CPU's cache directly. That'd be fun to make real!
Integer mode fans will never listen mp3's, however, we think VOX should play them.
So that's really serious question wether VOX should support Integer Mode - which hopefully makes VOX jump from "very good sound" to "absolutely good sound" for the price of having to spend lots of time adding and supporting it and making useless lots of stuff we did previously. However VOX has some Integer Mode attributes - Sample Rate sync that allows to bypass re-sampling and Hog Mode which makes device solely available for the app, effectively bypassing system mixer.