Cary MB CAD 500 amp

I don't understand the question? Do you mean why have they 'stopped' building this product?

Macd, Typically companies build\make improvement on existing equipment. For instance, Cary could have "Cary MB CAD 600 amp" or "Cary MB CAD 1000 amp".
This may include better power supply, 600 or 1000 watts version, bigger/better transformer etc. basically an improvement over the initial version.

Why does a company discontinue a product? In some cases because a new design exclipses the old design, or it just didn't sell well in the first place. I can't answer this for sure in the case of the Cary CAD-500MB, but it could very well be because it didn't sell well. So, if that was the case, why would they build on it with a 600MB, 1000MB, etc? With that said, when I sold Cary products, I really liked the CAD-500MB. They were very good monoblocks.

Perhaps the two-channel audiophile wasn't convinced that a primarily tube manufacturer could make a good solid-state amp (which would have been a mistake), and perhaps the home theatre buyers didn't want stacks of monoblocks to run their setup. Just guesses.

Everest, You make a good point. I think it probably did not sell, b'cus besides the review on 10audio, its hard to find other reviews on it.

Is this its replacement?

http://www.caryaudio.com/press/news.html

The new Model 1.600 is a mono power amplifier rated at 600 watts into an 8 ohm load. With a 1500VA universal voltage dual primary transformer, there are four independent power supplies within the Model 1.600, one for each rail of each amplifier channel. The power supply also incorporates inrush current limiting that is switched out after a short time period once the amplifier has been powered on. This allows the amplifier to not cause brown outs when switched on, but pull the high currents necessary for proper performance when in operation. On the audio input section, the Model 1.600 utilizes the THAT Corporation 1200-Series In-Genius™ high-CMRR (Common Mode Rejection Ratio) input stage which feeds the new National Semiconductor LME49811 High Fidelity Power Amplifier Input Stage. The LME49811 chip drives the final, balanced bias circuit and a triple output stage yielding an unprecedented low noise level, superb balance, and extraordinary power levels.