The Titans have 99db sensitivity. This means a clean 20w amp will push them to reference levels at normal listening distances. If you have a HUGE space and sit very far away then more power will be nice. Otherwise, it’s just not important with speakers like this.
XPA-3 would be perfectly fine unless you have an absolutely massive room (10,000+cuft), even then it may be enough depending on the listening level. For most people even a normal AVR powering the Titan-818's would likely be enough to get the output they require. The speaker can technically handle amplifiers of 1000+ watts but that would give you near earbleed levels in most rooms unless you are utilizing EQ to boost the bottom end to extract large subwoofer level performance from the speakers which is really the only reasonable purpose for an amplifier of that size with these speakers.