I think most hobbyists will use a paper back veneer. For that type, the best method is cold press - a pva veneer glue under pressure. A vacuum bag is best, but a bit difficult to use with speaker cabinets. We fashioned a hydraulic press in our shop to press flat surfaces. Whenever possible we buy pre-veneered mdf.
I have heard of others having good luck with the iron on method. Coat both surfaces with the PVA glue, let it dry and then iron it on. The heat activates the glue and then sets again once cooled. I think this is a bit more risky. If you miss a spot with the heat, it might start to bubble up later after you've got your finish applied.
It is much easier to use contact cement on this type of veneer, but I don't think there is a veneer manufacturer that will recommend it. This type of glue never hardens and with two wood surfaces, the material will shift and probably come apart at some point. You might not mind these potential fixes If it is your own project, but if you ship it off to a customer, you don't want it ever coming back.