I second Seroquel being one of the only things that works reliably for killing a stim trip (even a bad trip) or comedown. It's definitely one of the strongest hypnotics all-round, but it's probably not something you're likely to have on hand.
8mg of Xanax is not a lot for someone with a high tolerance for benzos or alcohol (even without taking the meth), and even double that probably wouldn't do all that much. Some people claim to have more success with Klonopin, which I haven't tried, but I don't see how it would be much different apart from a slower onset and longer duration. IME Ambien and its derivatives tend to make things worse (in some cases even hallucinations, amnesia, delirious states)... The only benzos I can think might really help are the stronger hypnotic ones, such as Rohypnol, Halcion, temazepam, nitrazepam, etc, or possibly some of the RC benzos like flubromazolam, but I doubt you'll be able to get hold of any of those in a pinch.