30 yr old speakers- time to replace drivers?

If you like the speakers and want to keep them, there are some things you can do as described above to keep them going another 30 years.  

If the tweeters have ferrofluid, it's worth refreshing.  FF gets thick or dries up, and affects performance.  

Parts upgrades are worth some consideration depending on the parts they used originally, and depending on your system, abilities, and desire to do so.  Caps from the 90s tended to last longer than caps from the 50e/60s, but there are a lot of excellent grade caps available now.  Cheap sand cast resistors and iron core inductors can also be upgraded with wire wound non-inductive types and air core inductors....stick close to original values.  Cheap speaker inputs and junk wire can be upgraded too.  You can replace push on connectors with direct solder if you go this far.  You can also bypass fuses and circuit breakers for better sound.   This is also a good time to check stuffing, bracing, insulation, etc.  Each of those are pretty subtle as stand along upgrades, but doing all them can have a big impact, and kick the performance up a notch or two.  

You might have to go on Audiokarma for vintage gear and repair of said gear.  It seems some here would report not to bother recapping or reworking any thing over a certain age. I guess we should put all our vintage gear into a landfill! 

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.

@pureclarity sorry, I missed the problem statement in your OP! do you hear distortions, or any kind of sound problems' to fix? If not, why to “fix” anything?

Speaker design (fix) can be very challenging, and needed some assessment, x-over sims, measurements etc, not available to everyone.

The Xovers are totally worn out ,having upgraded Xovers a long time the capacitors dry out for sure after 15 years or less , you loose detail and then uneven output per speakers ,then just stop working.