I managed to get a great spaghetti western tone from IK Multimedia's Amplitube Fender.
The trick I think is not lots of distortion but two layers (or more) of different distortion so you get a complex breakup. It's lofi all the way. Try going from one amp head into another then into a small speaker. Spring or plate reverb before the distortion and a bit of tremolo helps too. There is not a lot of compression used, just a bit of tape saturation.
Try Ferric TDS from Bootsy at the end of the chain:
http://varietyofsound.wordpress.com/downloads/
I'll upload some of mine it I get time. I love the spaghetti.