The NHT VT is a good loudspeaker, but it's not a special loudspeaker. I have a pair (VT-2, I think) in a bedroom mainly used for watching video and they work well. I would advise not putting much, if any money into them. NHT won't help you with any info on the crossovers or drivers. So unless you're a real DIY kinda' fella it's not worth the money to even figure out what you would need to change. As long as the drivers are in good shape you're pretty much there. The loudspeaker are what they are.
A true low cost change would be to replace the internal wiring.