sundara sound

why are you using them on a speaker amp?

the headphone section of a speaker amp is most times just slapped on and not really designed. the Sundara too is voltage hungry headphone

you need an actual headphone amp. this would do - SMSL DL100

Also they are rather linear in the bass region @pharmacylaw 

i didn't know that. I am about to retire and wanted to hear music without disturbing family members, and vice versa. so the headphone is not broken. But, then, why do the amps have a headphone plug in the front? What is the point of that?

They should play fine with your amp headphone jacks. Specs look to be 37 ohm and 94 dB for the Sundara.  I recently purchased a pair of Hifiman Ananda Nanos (14 ohm and 94dB)  and they play (surprisingly) fine directly plugged into a Samsung tablet, or a dedicated headphone amp (1.25 wats at 34 ohms) or the headphone jack of my MA6100 McIntosh integrated. 

You may have a bad cable?  Amazon has plenty of aftermarket, inexpensive cables you can pick up as a test cable. 

I'd contact Hifiman and see what they say.  The open box stuff has a std warranty I think. If they're faulty, they'll cover it. 

The Sundara is a really nice headphone, especially for the $$. Don't know why they aren't showing their wares on your set up but if you have a friend or relative with a dedicated headphone amp, plug them in and see what you think. Enjoy the music