I know over here in the US you can get used dt770s for $70. You might even be able to get new ones for that price over there. Highly recommend.

The VBSS seems like a fun build. Something that is a big up for me with the VBSS is the fact that you have to buy tools. Now you have a nice sub, tools for more diy speakers, and experience with those tools. Plus the pride of saying "I made that". You could always upgrade the driver later too.