I wouldn't use a condenser to record a guitar amp speaker. If you can, look into buying an SM57, it's a good all round microphone, and is durable and all that jazz.
For the low end hum, is it actually your amp making noise?
It can be caused by the amp being too close to electrical appliances, as previously stated, or your mains line isn't earthed, if I remember correctly.
The low end hum you hear as well can potentially blow your speakers, or severely damage them.
When I was last in the studio as well I had a nasty noise coming from my amp which was caused because I was using a stereo jack extension from my guitar to a mono jack lead into the amp. Because I'm right stupid. I took the extension out, had to play really close to the door, but it worked.
Hope some of that ramble helped a bit lol.