15 in full range drivers

Have you measured them? Could be a simple frequency spike somewhere. Though being full range could be cone breakup and that is not so easy to fix without adding another driver.
 

Not sure I would buy the Klipsch heritage speakers looking for low fatigue speakers. I found them a touch harsh at high volumes but I know others on here disagree. 
 

not sure your budget but these tannoys looks nice. 
 

I've had fatiguing issues and never was it my speakers.  Not saying it can't be but usually it is a component.  

That said, some speakers are a bit heavy on the high end because older audiophiles, the ones with money, tend to have hearing loss at higher frequencies.

Jerry

List your complete system (including room size and listening habits) for informed advice.

I have been using wide-band drivers for 20+ years and they are not for everyone/most.

DeKay

Was just looking at the Lil 15 inch drivers one click before this. Saw a rather favorable review.

As noted above, need more detailed info if you want a helpful response.