Yes, it can sometimes be a real problem to unscrew your old buffer tube. Using "Liquid Wrench" or "Kroil" to break the aluminum to aluminum "weld" will often--though not always--make it easier to take off. Kroil is better but may be hard to find. Whatever you use, let it sit with the product on it for a couple of hours before you try to unscrew the tube.
The flats on the end of the tube are a standard size, so you don't really need a special wrench. I think it's a 5/8" but don't recall for sure. Be aware that the flats will quickly round off if you use too much power on it. I've also heard some say they had good luck using a strap wrench to do this, but they don't work for me.
Good luck. If this doesn't work, get back to us. The "troubleshooting" area has some good info too.