Originally Posted by Silent Sound
What's your budget? The Royer 121 is probably the most popular ribbon mic for guitar amps right now, but this was posted in the Low End Theory, so that maybe out of your budget.
As far as an external preamp is concerned, it's gonna be hard to say. Ribbons do need lots of gain, but if you record your amps pretty loud, you may not need one. If you do need one, you might want to check out the Fethead or Cloudlifter. They are inline supplemental preamps that give you an additional 20 or so dB of gain to your old preamp. Of course, a good quality preamp will get you a better tone than the mbox will be able to on it's own or with the inline preamp addition. And with that you'd want better converters and perhaps a DI splitter so you could also take a DI of the track at the same time.... It can all get really expensive, really quickly.
My advice would be to see what you can do with the sm57. They've been getting great guitar tones by themselves for decades. If you want to try a ribbon, maybe try a Nady RSM 4 or RSM 5. They're good ribbon mics for around $100. If you want something nicer, get the Royer 121 and a Great River ME-1NV preamp.
Other things to think about...
I like to place my ribbon mics right up on the speaker, not as the room mic. For a room mic I prefer a small diaphragm condenser waaaayyyyy across the room for the BIG amp sound (assuming the guitarist is using more than one speaker) and I sometimes will use a large diaphragm condenser that does not hype the high frequencies back six or eight feet from the amp. I always walk around and listen to find what to my ears is a sweet spot and THAT is where I place the room mics.
On the speaker I will often get up close and listen for the sweet speaker--there is typically one that sounds a little better if it is a 2x12 or 4x12 cabinet.
I really love ribbon microphones on electric guitar--they will give you a fat, thick sound. I prefer them more for rhythm electric guitar more so than lead work.
For budget on ribbon microphones... This has been posted many times before, but I'll write it again: If you are looking for a ribbon mic in the sub-$1000 range, you have a lot of options. Most of them are coming out of China with the notable exceptions of the Beyerdynamic ribbon mics and the Oktava ribbon mic. Of the Chinese mics, there are basically three models and everyone is pretty much re-branding them. There is the long ribbon type, like the Apex 205 or the Cascade Vin-Jet. There is the short ribbon type, like the Nady RSM-4 or the Cascade Fathead. Then there are the dual short ribbon types like produced by Apex or CAD.
My suggestion is to find the best deal on a Chinese ribbon mic that you can. Look at the Nady RSM-4 or RSM-5, as already mentioned, or for a long motor type, look for an Apex 205. Look for others as well, but I suspect you will not be able to find a better deal than those.
Oh yes, I would be remiss in not mentioning a mic made by one of our fellow gearslutz: The DIYAC RM-5. Check it out here: https://www.me.com/gallery/#101734
Okay, so you have some ribbon mic information to chew on. Learn more about how they work, the figure eight pattern, the null areas of their pickup field, how fragile the ribbons are and all that.
I will not get into DIY ribbon mic stuff as I typically do. Look it up and you will find a lot of posts on the subject if it is something that interests you. And good luck!