Pierce, when you ask the world, you will get very diversified answers. I'll try and put them into perspective: a student horn (and in most cases the "intermediate" version) are built to be durable, easy to play and in tune. The durable comes at the cost of sound quality and projection. Pro horns are built with sound quality first, then intonation and easy to play. Many of them have all three of these in more than ample quantity! A used horn only makes sense if it is in good shape and plays well. This means that you buy from a reputable source, or play before you pay. The Yamaha that you mention is a solid horn that will last for a long time. The Yamaha 6xxx is a pro horn that is easy to play and if in good condition will knock your socks off! It is easily better for anyone that is mature enough to take proper care of it. A used Bach is 100% play before you pay. Mine rotted out from under me inside of 6 years. A new leadpipe and tuning slide were necessary. 6 years later, the next set as well as a valve job. I played this horn a lot. As far as the Kanstul for <$500, I am not sure. I think the Yamaha 4xxx is a safer bet! The bottom line is that NONE of us can take the decision away from you. I do suggest considering a used pro horn. You should play before you pay AND take a second set of ears that you trust (like a teacher) and compare that Yamaha to the pro horns. I am 100% sure in a blind test, many of the internet "experts" could not back up their posts. Good luck!