Hi,
Venkat all though you got the first part correct but some might've missed the other part

!! Specifically the part pertaining to the question that Bass King has asked !!
There is a difference between THX certification and THX modes found on the most modern AVR's or AV Pre's !! There are couple of modes such as THX Cinema and Music etc !!
What these basically do is to take the input singal (irrespective of DD or DTS ) and process it according to the complex algorithm that the AVR/Pre is programmed with and send it to the Amplifier,this also depends what the AVR/Pre is certified with,As in if the AVR/Pre is certified with "THX Select"(smaller then 300SFT) then the process will happen keeping that room size in mind and to make the input signal (DD or DTS) to sound best in that size room same happens to "THX Ultra" (Larger then 300SFT) !!
The in layman's term the THX Mode will reduce the extra brightness and such in the sound make it sound more natural in the Home scenario !! why this is done ?? it is believed that the sound mastered for Cinema has more dynamic range then the capability of the Home Cinema Equipments and needs to be corrected in order to get the best possible ( how far its true ? is left to one's individual capability

) !!
Here's an excert from the THX site on what they have to say bout these THX Modes !
Hope this info is enough for our Bass King to decide :lol:!!
Regards.