Willsenton R8 gets muddy When Too Much Sub Bass is Present

I just got a pair of Klipsch Heresy 4s after hearing them a few times and really liking the way they sound. Got them home, paired them with a Monoprice X2 power amplifier and enjoyed them tremendously. Even my wife enjoyed them enough to not be super pissed about the space they take up in our living room. Took the Willsenton R8 out of my office and connected it to the Heresys. When playing music that has a lot of sub bass my speakers take me back to being in my friend’s 95 Cavalier in high school. The bass drops in a song and the audio simulates an aneurysm. I have a Willsenton R8 set to amp mode connected to a Pioneer Elite SC-85 receiver as a preamp. Regardless of whether it is set to ultralinear or triode it sounds terrible in this scenario. The receiver’s settings regarding crossover have little to no effect also. This is all paired with a passive sub. With the sub off and the speakers paired with the R8 they sound like muddy crap.  Paired with the Monoprice X2 they sound amazing. This is definitely the amp and nothing else. I have tried using different sources of the same song, from record albums to FLAC, the X2 sounds great and the R8 massively underperforms. What could be the cause of this?