There are really 2 questions that have been asked. First, would you use a cheap dynamic or a cheap condenser to mic an amp? Secondly, what cheap dynamic or cheap condenser would you use? My answers (which lots of folks disagree with)- First, I would use a cheap dynamic for that crunch, and a cheap condenser for the "articulated rhythm". I would probably set up both mics and use one or the other, or a blend of the 2, in the final mix. I would also put a dedicated low frequency mic, such as AKG D112 or Audix D6, behind the cab if it is an open back cab (and reverse the phase).
Second question- an SM57 is a perfectly good cheap dynamic, but there is really nothing special or magical about it. You could use your AT, or AKG D770, or Sennheiser e835, or for wicked cheap, Behringer XM8500, or a host of other dynamic mics- My favorite is Sennheiser MD421, but it's *not* a cheap dynamic.
In short, MXL990 is one of the last mics on Earth I would put in front of a cab. An MXL V67G or AKG C2000B works rather well. As you say, the 990 is "bright". I would call it "hyped". When it comes down to it, I think the 990 is a very good paperweight or "marital aide", and there are other cheap condensers that suck a lot less- Studio Projects B-1 and MXL V67G, for example.-Richie