Headphones are about the worst thing you can plug random audio signals into; they have low impedance (so can easily overload the output stage), are pretty fragile, and typically have no protection, so should only be used with circuitry explicitly designed to drive them. I suggest testing with some cheap active computer speakers instead, if you have some nearby.
(I say “cheap” but more expensive ones may have even more headroom, but then you’d have to fully trust a random guy on the Internet and I’d be a bit careful with that 
)