Upright basses, or double basses, are often recorded with two microphones. One captures the string noise (attack) while the other captures the body (bass) of the instrument. When combining the two, you should be able to capture a usable upright bass tone. However, when it comes time for mixing, you might find that you want to make equalisation adjustments to bring out the best of the bass in relation to other instruments.
Rather than equalise the upright bass, adjust your microphone placement to emphasise or de-emphasise the different frequencies being captured. If using multiple microphones, try increasing the volume of one microphone while decreasing the volume of the other to create your desired bass tone.
Roll off the low end below 50 Hz. Most stereos cannot recreate frequencies below 50 Hz with accuracy, so this will not substantially affect the upright bass tone. Increase or decrease the "weight" of the bass by boosting or cutting in the 80 Hz to 100 Hz range. If the bass sounds "boomy," try cutting a few decibels between 100 Hz and 150 Hz. If the bass tone is lacking "warmth," boost the EQ between 100 Hz and 300 Hz.
Enhance the attack of the bass by boosting in the 500 Hz to 1500 Hz range. Doing so will increase the presence of the bass and allow it to stand out more in a mix. If you feel that your bass tone has too much attack, cut a few decibels in this frequency range.
Depending on your preference, string noise can be emphasised or de-emphasised by boosting or cutting between 2 kHz and 5 kHz. If boosted, this range can also add brightness and definition to the upright bass tone. Add "air" to the bass tone by boosting all frequencies above 8 kHz.