Is the NHT xd System worth buying today?

I don't know what you mean by 'still valid today' but I have heard the NHT Xd and would not hesitate to acquire the system if given the chance; advancement in technology notwithstanding. It's one of those things that stay with you for a long time, much like what the original Logitech Transporter did for me at first listen. I regret not jumping on the chance to acquire those kinds of components no longer in production.

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Its a killer system....esp for it's size

Half isn’t a great price considering I’ve seen them go for as low as $1200 for a 2.1 set on ebay, and $1500ish on Audiogon, with the XDA amps.

I wouldn’t pay $3000 for it today. First of all NHT ended up selling upgraded versions of the monitors (M60), and secondly the drivers aren’t really all that valuable, they were pretty much mid level Seas drivers at the time. The selling point of the system was the XDA amp, which at this point is pretty old. Secondly it was a notoriously noisy machine that was prone to mains hum and other noises, back when the XDs were in production people would pay to "upgrade" the XDA with upgraded caps and electronics to get rid of the noise.

Finally, the XDA is probably more hassle than it’s worth, and plenty of people have commented that it doesn’t really sound any better than the passive version of the NHT XD with a custom crossover which I believe Zaph and Selah used to build for people with dead amps.