Hopes this helps explain about THX.
THX is just a certification process.
Much like any other certification, it costs money to have your equipment certified, and for using the THX name on your products.
A reviewer once posted that many manufacturers do not seek THX certification because they feel that they can not afford the fees, or that the THX certification would greatly add to the selling price of an entry level, or mid level piece of equipment, and that the unit he was reviewing met or exceeded every THX parameter, but was not THX certified.
Makes sense, being THX certified does not automatically make a unit a better piece of equipment. Remember, THX certification is more than specific performance measurements, there are also specific features that must be included.
There is another issue, any certification process usually has a minimum manufacturing requirement... a company that makes design changes on an active basis (real changes, or just marketing changes so they can have a new model number), or does not make enough units of that model, may not be able to meet the manufacturing requirements, even though, the unit actually exceeds the certification requirements.
I am comparing several $1000 to $2000 receivers, and not one of them is THX certified.