I'm looking to buy a second hand head but I don't know which one is the best buy for the buck, any info would help be gratefully . -Genz Benz STM 600 head: 500 dollars. -Ampeg SVT 3 Pro Head: 750 dollars. -SWR 550 X Head: 500 dollars. -Mesa Boogie M3 Carbine head: 850 dollars. Thank you all in advance!!