That sounds like FFT data is what you want, which is available from BASS_ChannelGetData. For example, to get the level of a particular frequency, you could do something like this...

`float fft[1024];`

BASS_ChannelGetData(handle, fft, BASS_DATA_FFT2048); // perform a 2048 sample FFT

int bin=2048*frequency/samplerate+0.5; // calculate the nearest "bin" to the frequency of interest

float level=fft[bin]; // get the level of it

Please see the BASS_ChannelGetData documentation for details.