I realize this is an old thread, but I got here looking for some sort of bandpass filter for darkice. I noticed that I drastically reduced the bandwidth used and hissing noise by adding lowpass filters a while ago, but I noticed that now everything was too bassy especially if you listen to the stream on a THX 7.1 system.
I also wanted to share something I found that made my system a bit less complicated. Is the use of alsa and the dsnoop plugin. This allows me to split the stereo input in to 2 input devices. So the left channel is being fed by one scanner and streamed to a server and the right channel by another scanner and being sent to another server using a single sound card.
All you need to do is located on this URL: Issue 31 - darkice - Split Stereo input to TWO SEPARATE feeds - DarkIce is a live audio streamer. - Google Project Hosting
My streaming computer is an old wyse terminal VX0 I picked up on ebay for $25 with debian installed on a 2gb flash USB drive. The sound card is an USB iMic by griffin. Everything run nice and after enabling cpu scaling the machine seats idle at 532Mhz and according to my kill-a-watt is only using 4 to 6 watts. Beats my old setup by about 50 watts and $10 cheaper than a raspberry pi.
I just thought about shearing this info.