Yes, 150W from four current production EL34s isn't possible unless you use some creative definitions of power measurement. It was just about achievable in the old days with Mullards and Sylvania 6CA7s, although probably stretching it a bit even then. But it's amazing how much more "power" you can get if you allow, say, 10% THD. It's still just another way of BSing with numbers.
The circuit boards aren't bad - from what I remember they're the typical Behringer double-sided blue glass-epoxy type - that's not where the problems are likely to occur. The transformers will be their own OEM variety from some factory in China - no way are they going to fit anything with a name you would know, that would probably double the build cost of the amp at source. Transformers are indeed the most expensive components, although that's not always what matters in mass-production - the R&D costs, even divided by the number of amps made, can still end up being the single largest cost. Hence why Behringer can cut a lot on this, because they often just steal other companies' work so they don't have to do as much themselves...