What 's the different between MDF and HDF ?

Initially, I was also confused with these two terms HDF and MDF. HDFs are costlier than MDFs and HDfs can last for 15-17 years with laminations whereas MDF lasts for 10-15 years. So there are some basic differences in these wood and it completely depends on your usability which you should go for. 
However, I got a good article on HDF and MDF differences. Maybe it is useful for other readers as well. :)

https://sawauthority.com/hdf-vs-mdf-wood/

I think you should be able to find sheets of 1 1/4" thick 4'x8' if you shop around I've used it on commercial buildings as subfloor. sorry I don't remember the supplier.

While MDF may have a 10 to 15 year life span in floors where it would be under constant pressure.  That is not the case in speakers.  I have speakers that I built and laminated 40 years ago that still look and sound great.  
What was posted is not applicable for speakers.