There are wide differences in amplifier build quality. A lot of the cheap ones are, well, cheap. If you're cool with a third world el discounto power transformer smoking off after a few years (and assuming you have proper insurance coverage and, hopefully, a sprinkler system and fresh batteries in your smoke alarm), then you can save plenty of money. If you want an amp built correctly with reliable parts, you will have to spend some money. Further, the cheap tube amps are on PCBs, which can be difficult or impossible to repair if something goes wrong.