Low Powered Tube Amp Distorting High Efficiency Speakers?

There may be a simple answer to this, but I can’t seem to find anything in my online searching. I have a pair of Zu Soul 6 99db efficient loudspeakers connected to a 15 watt Leben CS300xs integrated tube amplifier. When I crank up the volume to about 10 oclock on the volume dial, the speakers get pretty loud, but I hear a bit of distortion when certain high frequencies come into the recording.

My understanding is clipping / distortion occurs when the amp is driven too hard, but the volume being not even at halfway around the volume dial, I’m wondering why there would be distortion. Granted the Leben’s 15 watts gets pretty loud at 10 o’clock on the volume dial through the Zu’s. If anyone can explain why this is happening I’d love to know. Below are the Zu specs:

Impedance: 8 ohm nominal, 5.1 ohm minimum, balanced load.
Sensitivity: 100 dB-SPL @ 2.8V, 1m ground plane

Bandwidth, In-Room, Power Response: 38 - 28k Hz ±3 dB within 10 deg of axis at listening position greater than two meters.
Horizontal Listening Window: 45˚ @ -6 dB
Vertical Listening Window: 45˚ @ -6 dB

Group Delay: <5 ms

Max Power: 150 watts (full bandwidth)
Max Power LLF: 400 watts RMS (80 Hz high-pass, 12 dB/octave. Assumes power is unclipped, amp rated at 500 or more watts @ 8 ohms)
Avg Room/Moderate SPL: 2 - 4 watts
Avg Room/Loud SPL: 5 - 20 watts
Avg Room/Big Concert Levels: 20 - 150 watts

Thanks!