Yes, BASS_MIXER_FILTER did have effect on decoding channels. Mixer sources need be decoding channels, so it had to
The new BASS_ATTRIB_SRC attribute applies to decoding channels in the same way as the BASS_ATTRIB_FREQ/PAN/VOL attributes. BASS won't apply any processing for those attributes itself on decoding channels, but BASSmix will read the settings and apply them when generating mixer output. Here's an example of setting a mixer source's SRC quality...
source=BASS_StreamCreateFile(FALSE, filename, 0, 0, BASS_SAMPLE_FLOAT|BASS_STREAM_DECODE); // create a decoding channel from a file
BASS_ChannelSetAttribute(source, BASS_ATTRIB_SRC, quality); // set its SRC quality
BASS_Mixer_StreamAddChannel(mixer, source, 0); // plug it into a mixer