Just curious: why do you want the 635a? I have four RE-55s. They're similar, but with no bass attenuation, so you can record instruments with low-end, too. Also, the high end goes a little higher. The RE-55 is my Swiss army mike... I have to have a good reason to use something else. Of course, they are a bit less common and have been out of production since about 2000. :-(
Otto