The NS-500 was released in 1983 and is a ported two way whist the NS-500M is a three-way incorporating a graphite mid-range transducer. In the early 1990's Yamaha released the Japan-only YAMAHA NS-500YST, showing some of the character and style originating from the ultra hi-end Diatone top models. One major difference with the YST tweeter is that it was produced with high concentration Be deposited on a titanium dome, versus the earlier top model Yamaha speakers which were Be alone.
About the tweeters: Not all Yamaha Be drivers are the same or equal. A good example is the NS-1000X uses tweeters and mid-ranges that do not perform to the same standard as those of the other high NS Be models, also the driver placement compromises the transfer function of the overall speaker to a great degree (besides the point but good to know).
I noted in another thread that Yamaha's drivers are similar to some AR drivers in respect that they share the issue of not being able to dissipate heat very fast. It's not an issue for the people who listen at 80dB, which is already quite loud anyway. The chances of finding them with drivers that do not have browned leads is quite rare unless they were taken care of; problem is everyone says they are taken care of but we know that is not the case. What happens is the leads brown and they can still pass current and function in most cases, but they sound rough due to distortion which is an inherent property of this resistance in the leads. They can be repaired. One sign of damage is when taking a response measurement the tweeter may take a sharp dip of a few dB at about 7 to 10kHz or be really peaky. Other measurements will show the problems too. Sound wise the distortion may be mild to severe and induce irritation.
This probably didn't answer your question entirely but maybe that info might be helpful. In other words a set that have never seen more than a few watts will we worth a lot more than the normal going price to someone who knows what the speaker should sound like. I can relay manufacturer specifications if you want.
About the tweeters: Not all Yamaha Be drivers are the same or equal. A good example is the NS-1000X uses tweeters and mid-ranges that do not perform to the same standard as those of the other high NS Be models, also the driver placement compromises the transfer function of the overall speaker to a great degree (besides the point but good to know).
I noted in another thread that Yamaha's drivers are similar to some AR drivers in respect that they share the issue of not being able to dissipate heat very fast. It's not an issue for the people who listen at 80dB, which is already quite loud anyway. The chances of finding them with drivers that do not have browned leads is quite rare unless they were taken care of; problem is everyone says they are taken care of but we know that is not the case. What happens is the leads brown and they can still pass current and function in most cases, but they sound rough due to distortion which is an inherent property of this resistance in the leads. They can be repaired. One sign of damage is when taking a response measurement the tweeter may take a sharp dip of a few dB at about 7 to 10kHz or be really peaky. Other measurements will show the problems too. Sound wise the distortion may be mild to severe and induce irritation.
This probably didn't answer your question entirely but maybe that info might be helpful. In other words a set that have never seen more than a few watts will we worth a lot more than the normal going price to someone who knows what the speaker should sound like. I can relay manufacturer specifications if you want.