I know back then THX was mainly just a logo that companies would pay for to show that equipment is up to THX standards however, now I see more than logos, it seems like THX is actually a feature that can be used in AVR's like THX PLUS, SELECT,ULTRA etc.
Is there really a big difference in sound with THX capable equipments?
THX is primarily a quality assurance program. THX certification guarantees that a piece of equipment meets certain minimum standards, which vary depending on the certification level. Most, but not all, of the details are kept secret, although some have been deduced. (THX Ultra amplifiers have to be stable when driving a 4 ohm load, for example.)
Certification costs money, so many manufacturers are not bothering to get their equipment certified these days. Many customers argue that the price increase caused by THX certification isn't worth it, either. Others disagree and are willing to pay the premium in order to have the reassurance that the equipment they've bought is high quality.