you can do an "off-the-air" experiment to test the effectiveness of such a filter combination.
1. Record a 12 kHz IF signal with interference as a wav-file with 48 kHz sampling rate.
2. Run this wav-file through DL4YHF's Spectrum Lab program (freeware) and apply the filter function that you want to test. Let it write the output to a new file. Spectrum Lab may be found here
3. Run Dream using the commandline parameter -f once for the original file and once for the new file and compare results.
I have done so using the latest Dream CVS version and a notch filter on the centre frequency, but it did not improve decoding. The result may however depend on the kind of interference encountered. (It did improve reception with the last version of Dream, but not with the current one, which has a better channel estimation.)
On a sufficiently fast system, it might even be possible to do this all in real time by cascading both Spectrum Lab and Dream by means of a program named 'Virtual Audio Cable' which can be found at
http://www.ntonyx.com/vac.htm but so far, I have not tested this.
Edit: I am still searching for a freeware program that allows to cascade audio applications like 'Virtual Audio Cable' does, but I haven't found one yet. If anybody knows of one then please tell me.
73, Roland, dk8cb