Low Powered Tube Amp Distorting High Efficiency Speakers?

Another possibility is the recording itself. Often vocals are VERY closely miked which can magnify the sibilance. This is true of many rock, pop or new country recordings. Add to that the "loudness wars" recording style that still hasn't gone away in many recordings, or a variation thereof where some popular recordings are mixed and equalized in a manner that makes them stand out when played on so-so equipment like car stereos, cheap radios, ear buds and the like. These effects can really sound nasty when played on a full-range stereo at louder volumes. 

So, the question is: does this problem happen with all recordings or just certain ones? If just some recordings, I think the above comments might explain the issue. If all recordings, maybe it is the speakers, or the acoustics of the room, or a problem with the amp. However, don't forget to look at the source, whether phono, CD or streaming digital. Could be you have the wrong cartridge or phono preamp for your system, or a nasty sounding CD player or DAC. 

Actually that's a good point, hopefully its the tubes or even the source thats causing distortion. Try replicating the problem with a differert amplifier if you have something else available on hand.  If it still exists, it could be the speakers, but you certainly want to isolate the problem first to figure out where its coming from.   

I have a good old fashion Dynaco ST-70 that starts sounding distorted right before the tubes go.  Might try a whole new bank of tubes. 

Thanks for all the feedback everyone. My source is a PS Audio DirectStream Jr DAC with Tidal. The distortion is not necessarily in the recording, but I’m pretty sure its not a bad driver. Gonna get a bit specific here, but I can hear it in the louder passages of Caroline Shaw’s No. 2 Sarabande on her album Partita for 8 Voices (an incredible recording of her Pulitzer Prize winning album with Roomful of Teeth). Towards the beginning one of the sopranos on the left of the soundstage raises her voice a bit and the sound distorts. When I switched the speaker cables I heard the distortion on the right speaker, so its not the driver.

On reading your responses, I thought maybe I have too much gain coming from the DAC (I usually run it at 100 % volume and use the volume on my Leben to increase and decrease the sound), so I turned on the analog attenuator on the PS Audio (which drops the signal by 20db) and that definitely seems to have helped. I now have the volume around 10:00 for normal listening levels, and I found the sound overall got better - maybe its due to a few more watts pushing through my system when I’m listening at normal listening levels? I definitely heard a change - just a bit more depth of soundstage, fullness in the sound, clarity. But maybe just a slight bit of decrease in dynamics (though not as peaky which Zu’s can be from time to time). So I may have fixed the problem. However, when listening to classical tracks, it seems I may not be able to get the volume I want to really feel that immersive experience. Still playing around - gonna see what happens when I turn the attenuator off and just lower the DAC volume to around 70%.