Hey, the Champ wants a 4 ohm load if I'm remembering right - I doubt the C. Rex is 4 ohms, right? If so, you might be losing a little volume with the bigger load, but I wouldn't think enough to merit it being down so much as you're making it seem. Is your 10-ft cable a speaker cable or a guitar cable? Speaker cable has fat conductors for both + and - current paths, guitar/co-ax cable generally uses very light gauge wire for its signal conductor which can be problematic. Also, you have the potential for high-impedance connections at the adapters. So, yeah...I guess I didn't really narrow it down much for you, huh?
I will say that a dimed Champ should be able to make a very loud noise with a properly hooked-up 4 ohm 12" speaker, even one that's not terribly efficient. If you're getting almost no sound or very quiet sound, be careful and make sure your output transformer isn't overheating!