Play a bass guitar through the bass amp and check for any rattles or buzzes.
Tighten or loosen screws on the cabinet in order to fix any buzzing or rattling sounds that were discovered during step one.
If the bass amp is small enough, elevate the amplifier one to two feet off of the ground in order to damp the signal. Placing the amplifier on chair is a commonly used method in many recording studios.
Place dampening material on the floor directly in front of the amp in order to prevent the bass signal from reflecting back up into the microphone. For an inexpensive solution, you can use bath towels as your dampening material. For best results, you may want to consider using professional sound dampening products that are specifically designed for sound recording. Professional dampening material is a special type of foam. These products are usually referred to as dampening foam or sound absorption foam by the manufacturers.
Choose a microphone that will properly capture the bass signal. The Shure Beta 52A and the AKG D12 microphones are specially designed in order to pick up low frequencies. If you do not have one of these microphones available, a Shure SM7 can be used, however you will lose some of the lows.
Listen closely to each speaker of the bass amp. Listen to find the best sounding speaker with the clearest tone. If the bass amp only has one speaker, listen for the "sweet spot" that will be the best place to position the microphone.
Place the microphone about 3-6 inches from the speaker you have deemed the best candidate for the recording.
Record. Record a bass guitar through the bass amp and experiment with microphone positioning until you find the best possible sound.