HF Audio filter bandwidth comparison
Much ado is made over purchasing optional filters for HF radios. Many radios today come with hardware 2.4kHz filters, with options for 1.8 and 2.1kHz filters. Other radios such as the Icom IC-756, Kenwood TS-2000, any of Ten Tec's current radios, etc. are mainly digitally based (of course, some of these have additional hardware filter options e.g. Ten Tec Orion II).
My personal experience over the past decade has been that a 2.4kHz filter is best for the average amateur's all around use. I have operated SSB in contesting as narrow as 1kHz in SSB (with a low cut of 700Hz and high cut of 1700Hz) at times. However, when I had gone that narrow, often the IM and other interfering characteristics (splatter) of the offending transmitters were usually the dominant factors--no hardware filter available to amateurs can remove this effect.
In my opinion, if someone is limited to a non-digitally based radio with only hardware filtering, and has interest in contesting, they may want the 1.8 and 2.4kHz filters. The 2.1kHz doesn't make enough difference in my opinion to be worthwhile for the average amateur.
The audio file below contains audio from one station over about 1 minute sampled at various filter bandwidths. The station being received was on 80 meters SSB, at about S7 on a Kenwood TS-2000. Notice how very little difference is audible when going in 200Hz steps. The bandwidths vs. time are as follows:
(times in seconds)
0.0 to 11.5 1600Hz
11.5 to 22.0 1800Hz
22.0 to 34.0 2400Hz
34.0 to 42.0 2600Hz
42.0 to 52.0 3000Hz
52.0 to 102.0 5000Hz