Just because your speakers can go down to 20Hz, doesn't mean you should make them if you have capable subs. Those CVs are fairly sensitive. At a 2m distance you will need less than 64W to drive them above reference levels full range. Any decent receiver can do that. Buying an external amp will be a waste of money for no audible difference. You say you have a HSU VTF-3 MK4 sub and will be getting another later on. The money wasted on the external amp should go to the second sub now. You'll get far better performance running the CV's small crossed over at 80Hz over to the dual HSUs.