a] Can a DIY hobbyist duplicate the sound of an expensive speaker?
Easilly providing you have the right information. If you can get the drivers and a detailed schematic all you need to do then is copy the cabinet. People build clones all the time. There is a big market in JBL Vintage Large format monitors from the 70's to early 80's. Over on Lansing Heritage we have had literally dozens of these systems cloned in both the US, Japan and Europe. I have a cloned pair of 4344's I built about 8 years ago. The biggest issue now is the availability of cone kits. Up to several years ago most of these were readily available so you could purchase driver cores and have them reconed. You would end up with essentially a brand spanking new speaker. That what I did with my 4344's all the drivers were fresh recones and not 30 year old originals. Now those kits are drying up.
Rob