Manufacturers have the option of deciding if they want to APPLY for this certification, steps are spend $30-$100k ( depending on the product line dept etc ) send the Lab products for testing, ( a 3rd party certification center ) wait for them to test for sound quality, processor quality etc and the product either passes or fails, if it passes ( which a ton of high end receiver could but like others said, the choose not to spend the $$$ ) then the mfg can now put the THX badge on the box and products and charge us a premium for it. THX receivers will get you a certain level of sound quality, no doubt but keep in mind there are dozens of other avr's that could most likely pass the test but never were submitted. On another note, I would doubt that any AVR's in the $200-$400 price range could be approved for a THX rating
200 +++ BD discs and counting...still enjoying 60+ HD-DVD's too!!