When you're learning new songs, EQing for the specific instrument that you're learning can be challenging. With music mixed down into a single file, there's simply no way to truly isolate the part that you want to hear (see below for an exception).
For when you first start trying to EQ out other instruments and EQ up the one you're interested in, bass guitar is about the best place to start. There are several reasons for this.
First, bass is non-directional. That is, when you hear sound at the bass end of the frequency spectrum, you cannot tell where it is originating from. This why when you get a new stereo system, the setup instructions tell you that you can put the subwoofer anywhere. Subwoofers carry all the LFE (Low-Frequency Effects) sound in movies, and all the bass in regular music. That sound ranges from about 3 Hz to about 120 Hz. 120 Hz is about B2, which is an octave and a half-step below middle C (C4). Which leads us to the second reason...
Secondly, since bass is non-directional, it is generally center panned, just like vocals, making it easier to locate the sound in the stero mix.
Now, depending on the software or audio system available to you, you'll be able to filter better. Ideally, you want to have a precise EQ with as many bands as possible. You'll also want a stereo EQ as a mono EQ is very limited, especially for filtering out and filtering in different instruments and frequency ranges.
For bass guitar, first level down the Gain to -15 dB to prevent clipping. Next, boost one side of your stereo EQ by 10 to 15 dB in all the bass frequencies up to somewhere around 120 Hz to 150 Hz. We'll use the left side EQ for this example. (It doesn't matter much whether you EQ the left or right side.) Leave the right side flat. To get more bass guitar "punch", go up another notch or 2 in the EQ to 220 Hz or so, leveling them down to the rest of the EQ at about 10 dB and 5 dB.
Depending on the music that you're using, you can continue to slope down from the bass frequencies closer to the mid-range, but don't raise any EQ settings above 400 Hz. This is the upper range for a bass guitar. If your music has a jazzy bass guitar with lots of slap in it, the harmonics go up to about 5,000 Hz. You can EQ in the 400 Hz range to 5,000 Hz, but you will be affecting many other instruments as well, and the value of that EQing may diminish. This is strictly a "trial and error" thing to try out.
If you're listening at this point, you should easily be able to hear more of the bass guitar in the left stereo channel. If you can, use headphones and not speakers as it will be much clearer.
Finally, to get rid of the vocals, invert 1 of the left or right stereo channel, but not both. Add the 2 stereo channels together into a single mono channel, and you will hear the vocals removed while the bass is now much easier to hear.
I've never heard of a stereo that can actually do any of that last step, so you'll need to do it in software. Any decent audio editor will work, and you can even use Audacity. Basic audio editors that are primarily for home movies don't have the necessary features to do this, so you can't use something like Windows Movie Maker's audio editing.
Depending on the stereo mix, you can mix part of the left and right channels into each other to gain greater clarity. Try a mixing something like 25% of the left channel into the right, or vice versa. Remember to reduce the Gain to compensate as this increases the output and could result in clipping.
To help you see some of these settings in action, I've uploaded a *.gdt file with the settings describe above for a loop in Steve Millers' Take the Money and Run (click to download). You'll need GDT to open the file, and you'll also need to have the MP3 as well (or any other compatible music file). If you don't already have GDT, you can download a free 15-day trial here.The settings are all as described above. Make sure to load the loop then go to the Channel Mixer and EQ.
I hope that helps some people with how to EQ for bass guitar.
Cheers,
Ryan
* Melodyne Studio offers DNA Direct Note Access which lets you isolate instruments and even individual notes. However, this is a $700 professional suite for studio use, and isn't suitable for general consumer use. The purpose is to correct mistakes in music that is for publication.