Mic a Bass Amp

There are USB mics and that would be an economical and simple solution.
 
I think the majority of us have some kind of audio interface we plug mics into.  (For example, I use a Roland VS-700 but also have a MOTU.)  You can get simple but high-quality audio interfaces for under $200 and it would be a very wise investment so you can use any mic you want.  Here's a well-reviewed interface for under $150.00.
 
https://www.sweetwater.com/store/detail/Scarlet2i2G2?product_id=Scarlet2i2G2&campaigntype=shopping&campaign=aaShopping%2520-%2520Core&adgroup=Computer%2520Audio%2520-%2520Audio%2520Interfaces%2520-%2520Focusrite&placement=google&adpos=1o1&creative=93365752201&device=c&matchtype=&network=g&gclid=EAIaIQobChMIgJfcqObD1QIVSFx-Ch1fVQIREAQYASABEgIm__D_BwE
 
For the record, I have an ElectroVoice RE20 mic that is great on almost anything: vocal, bass amp, kick drum, guitar amp, etc. but you don't need to spend that much ($449) unless you just need a solid all-around dynamic mic. If you get one, be sure to check the position of the bass roll-off switch if you are not recording kick or bass.  That said, tons of hit records have been made with the Shure SM57 or SM58.
 
If you have an audio interface, you can plug the bass in directly and use it like that, or run it though a plug in like Guitar Rig to get an amp sound.  In fact, even when I mic a bass amp, I split the signal and get a direct input as well so I have options later in case I decide the amp is noisy or doesn't fit in the mix..  I would say that 75% of the records I've made with a live bass player used direct input bass rather than an amp into a mic.
 
Sorry for the long answer--there are just a lot of options.
 
 
 
 

A better option would be to get a proper USB audio/MIDI interface as konradh says.

Most USB mics aren't outstanding - I don't think the SM57 or 58 is even available in a USB configuration. And if you get a USB mic now then get an audio interface later you won't be able to use both at the same time in an optimal configuration either.

Every time someone uses a USB microphone, A part of me dies.
 
 
A small investment must be made. A soundcard.
I know its tough because we spend a good amount of money on software and think we are ready to go,
but the soundcard is (in my opinion) the most important thing next to the software itself.
 
I have been pondering Mic'ing a bass amp for sound reinforcement.
I've been going direct inject for a few years now and im looking to expand my horizons.
But for most applications, going direct in to your soundcards pre amp will be more than enough.

audio interface is a must.
I've have been using Sennheiser e906's for instruments and love them.  Clean, good rejection and inexpensive.
 
rafone

"Every time someone uses a USB microphone, A part of me dies" Chuckie
 
Agreed completely.  
SM 57 is a go to on a guitar cab.  A Sennheiser 421 microphone also works well on guitar amps and is low cost.  With Bass - I either go direct or use a large diaphragm condenser.  That requires a pre and phantom power.  
 
I would never use a USB mic on something I cared about, other than maybe a podcast.  

...LOL! Got it! No USB mics!
 

If you'll be recording bass and guitar, have a look at the Shure SM7b, Sennheiser MD421 mkII, and EV RE20.
Some of the other "guitar" oriented mics won't capture a lot of bottom end (ie: SM57)