what mean by modular..?

The junior models of both VX and TX series are manufactured by Seasonic while the senior models come from Channel Well.

There is no fundamental difference between the VX450W and VX550W models of the VX series. The former is somewhat better in terms of noisiness and the latter, in terms of electrical parameters. Both models are good, so your choice should be based on personal preferences.

The difference between the TX650W and the TX750W is big, however. The latter is average in terms of noisiness while the TX650W tested today is virtually silent at loads up to 50% from the maximum. It will be a perfect choice for people who want to assemble an advanced PC configuration as well as for everyone who loves silence. Of course, this PSU is more expensive than others of its class. By the way, paying extra for the TX750W doesn’t make sense because it is noisier.

The highest-wattage model was a bit of disappointment. Like Thermaltake’s power supplies based on the same platform, the HX1000W could not deliver stable output voltages. As a result, it could not deliver the promised 1000 watts of output power, either.