USB Bit Rate Puzzle AMR Off-ramp Amarra Mac

I am equipped with the latest hardware and software, but the bit rate puzzle bothers me -- in the "Audio Midi Setup", I see "AMR DP-777 USB 44.1kHz 24bit" or "Empirical Audio Off-ramp4 USB 44.1kHz 32bit", I can change sampling frequency (44.1/48/88.2/96/192) from the dropdown menu with each device, but can't change the bit rate to 16 bit. Amarra's preference setting allows choice of different bit rates (16/20/24) with two "auto dither" options (TPDF and mBIT+); but these choices do not seem to change the 24 bit for AMR or 32 bit for Off-ramp4 (in Mac's Audio Midi Setup).

Most of my files are 16/44.1 apple lossless format directly from my redbook CD, how can I make all the software and hardware do 16/44.1 all the way?
Because it doesn't matter in that if you are sending less than 24 bits such as in the case of RB the system will simply pad the other bits. Trust me, it is the same as if you could select 16 bits. Same thing with the Empirical audio, if you send 16 or 24 bits it will simply pad the remaining bits. I suggest you do a wiki search for digital bit padding. Hope this helps.