15 in full range drivers

Seems to me that a single 15” driver would have a hard time reproducing the upper treble range. 
 

And I agree with others that SET amps are quite capable of beautiful full range music reproduction with the right speakers. Devore as one example. 

@corelli as James alluded to in the first post, I suspect the fatigue OP is experiencing is from cone breakup.  His 15in full range driver is literally bending as it tries to vibrate at higher frequencies in the middle and lower frequencies at the outer edges creating phase issues and distortion.  The distortion becomes much more noticeable as you turn up the volume leading to a harsh sound.  The driver is probably made out of paper to keep weight low to improve efficiency, but that comes at the expense of rigidity.  This isn't solvable with component matching, he's already using a low power amp, it's about song or music matching.  Pick stuff lacking in complexity, like slower tempo vocals with maybe a single instrument backing so it's mostly midrange and you get an excellent result.  When stuff gets complex it will fall apart, especially at higher volumes. 

Seems to be spot on will all the posts.  Thankyou.  When things get complex at higher volumes it is a mess. They are 15 in anilco's from Common Sense AKA Audio Nirvana. Unbelievable with solo's and slower tempo vocals and a great speaker, Old Aretha Franklin sounds great.  But I do like my 70's rock too Pink Floydcan be harsh. I am sure the upper midrange is the problem and why I think a simple EQ will help - which I will probably do just because.  I'll let you know how it goes.  But most likely just a step before I switch the speaker out...

complex music at high volumes can be fatiguing, even at the symphony. But maybe it varies greatly by the individual. 

From what I'm hearing (from my sealed box TangBand W8-1808 single drivers with dual SVS SB1000 pro subs), full range single drivers have opened my eyes/ears to a more liquid sound stage and depth i haven't heard before. Truly unbelievable for just one driver, but to only 70 db. More volume than that and, ya, they start braking up. I don't want louder than that anymore so, happy camper here.