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






My 50 year history in high end audio primarily comes from the massive power / current amplifiers in the beginning, working down in power to the 70wpc of triode tube amplification I have now. But what you describe is classic under powered.

@hertzhead +1. My JBL 4435’s are only 96 dB @ 1 watt 8Ohm’s, with 2-15’’ woofers and a horn.


1000 might mean nothing. What is your source? Some sources are higher level than others. that said, usually your amp won’t be the top of the range at 1000.

Every amp is different but I drive my 96 dB speakers as loud as you want with 2 wpc EL84 amp.

Do you have a spare set of tubes? Can you swap them out and see if it is any different?

Is it in both channels?

Finally, you mentioned "when certain high frequencies come into the recording". This is happening on more than one song isn’t it? it could be the distortion is recorded into the song and you’ve never noticed it before.

and one more thing, consider the possiblity of a bad driver in the speakers.


Your comment on the distortion showing up in certain high frequencies makes me think it’s a bad driver.  Things like cymbal crashes will sound a bit fuzzy and not crisp. One of the negatives of some high sensitivity designs is that they tend to be less tolerant to clipped signals.  Being just 15 watts I’m guessing you may have had an aggressive listening session or two and pushed the amp, which clipped the signal creating excess friction and thus burned the coils. I hope I’m wrong because replacing drivers isn’t cheap.