Recording bass direct is a very popular method. There are
several ways of going direct, but basses are generally not plugged
directly into the mixer input. Doing so usually creates an impedance
mismatch that loads down the pickups and affects the tone. You can use a
passive direct box, which is essentially an impedance matching
transformer. There are also active direct boxes. These are basically a
simple preamp. Some preamps have solid state electronics, while some
have tubes. Some preamps offer nothing more than adjustable gain stages
and a small EQ, while others have compression, delays, reverbs, flanges,
and speaker simulators.
Micing a bass amp is very similar to micing a guitar amp, but there are a few other things to consider. One of the important things is speaker size; a 4x10 sounds considerably different than a 1x15. The smaller speakers have better defined low end. You can hear the dynamics of the player much better with a cabinet with the smaller speakers, but there tends to be a lot of midrange that some players will object to. This midrange hump also takes up valuable space in the mix, taking away from the guitars and other instruments. The 1 x 15" will produce deep bass much more power than the 10" speaker will give. A 15" has a higher possibility of getting muddy, especially with more articulate playing, because the large cone reacts more slowly than a 10".
Allowing the transients produced by the bass guitar to sneak past the compressor might produce a great sound, but it's those same transients that cause recorder overload problems, so having an independent limiter section following the compressor is very desirable. The limiter threshold should be set just below the overload level of the recorder so that limiting takes place only on very loud peaks.
When recording the bass guitar, most engineers use a degree of compression. This is a good idea for a number of reasons. If the player uses a slapping or pulling technique, the note attacks can be very loud, so if you don't compress or limit, you either. Technically, the frequency range for most speakers and amplifiers on the market today is 20 Hertz (Hz) on the low end and 20 Kilohertz (KHz) on the high end. Most people's hearing range falls between 40Hz and 16 KHz and in fact, the specified frequency range of FM radio is 50Hz to 15Khz.
The reason for this excess range is 'headroom' necessary for the
prevention of thermo-harmonic distortion. The room that you are
recording in is the final factor in determining what you will hear when
you record your music. Any acoustic problems here affect the sound
reproduced, and thusly, your ability to interpret what you hear. The
amp, speakers, and the room they are in, all make up the listening
environment. When your mix sounds great in the studio and terrible
everywhere else, you know something is wrong.
We all can't afford professional acoustic treatments for our home recording areas (no matter how much we would like it.) That being said, here's the trick to mixing your music so it sounds good on systems other than the one you recorded it on. Do your mix using a near-field monitor system and keep the SPL (sound pressure level) down. This will decrease any negative effect that the room may have on your ears.
When recording a bass guitar amp, typically you'd use a dynamic cardioid mic, but a slightly more distant condenser mic can give good results too. It helps to raise the bass cab off the floor a bit with a cushion to stop the room resonances you sometimes get. Also, make sure you don't turn the amp up too loud, as the low frequencies overload mics pretty easily. You might have problems with 'standing waves' creating a boomy sound. These are frequencies that have wavelengths that are multiples of your room's dimensions. You need some form of bass trap to absorb these low frequencies. The best positions for these are corners. Blankets, drapes and normal foam won't work well because they aren't dense enough to absorb low frequencies. Also note that foam and cardboard egg crate is very flammable and should never be used.
Mixing a mic signal and a direct input can give really nice results. If you have enough tracks, record each one to its own track so you have more possibilities when you mix. You can combine the two to a single track if you have to. Use a stereo compressor for these tracks so the gain reduction is consistent on each channel. Another important thing to accomplish when recording is trying to get a good sound without using much EQ on the console. If you overuse EQ it doesn't sound natural. If you spend the time to dial in your equipment you'll only need to adjust the EQ's a little to help it sit in the mix. A bass roll off around 100 or 150 Hz is important to keep the low bass defined, and a slight boost around 1.5 kHz will add some finger squeak (which sounds good in moderation).