So, a few months after buying this amp, I finally decided to open it up to remove the USB LED shunt. The reason is that the Cantate USB LED lights up when connected to my laptop, even if the Cantate is turned off (USB LED is powered from the laptop). The only way to turn off the LED is to remove the shunt.
While I had my Cantate opened up, I also decided to finally try lerio's advice and change the gain setting to low by placing the three provided shunts in the appropriate positions.
That was half an hour ago. I've been listening to USB audio that first half hour, I changed to my Arcam just now. My headphones are still the HD650. I can tell you that even on low gain, the Cantate delivers more than enough volume to drive the HD650. I have it on 12 o'clock now. The Cantate volume knob goes from about 7 o'clock to 5 o'clock. More than 2 o'clock on low gain with the HD650 is really too loud for my ears. So no problems in the volume department.
I think the Cantate does improve if the gain is changed from high to low gain. However, my impressions might be clouded by expected results based on the impressions of others, i.e. placebo. I will listen to it a lot more on low gain though, I have decided to keep it on low gain for now.
I hope to get a stand-alone USB DAC soon to compare it with the built in USB DAC of the Cantate.
For now, here are a few more pictures of the Cantate's insides, of me installing the low gain shunts and removing the USB LED shunt. It took a pair of tweezers to place the low gain shunt closest to the housing of the Cantate.
(thumbnails, click for full size)

