Something to know about the HD650 - they're a nominally 300 ohm load, but they hop to 500 ohm at around 100 hz.
https://www.innerfidelity.com/images/SennheiserHD650.pdf
That kind of jump is a prescription for a headphone amp with an output impedance of maybe 30 ohms tops. A lot of headphone jacks on stereo gear have very high output impedance, 150 ohm isn't uncommon, and I have a vague memory of at least one being measured as high as 400 ohm.
You'd be surprised how high output impedance is even on recent, fairly reputable integrated amps. There's a great explanation of why picking 1/8 of your cans' nominal impedance (or less) is important for sound quality at
http://nwavguy.blogspot.com/2011/02/headphone-amp-impedance.html
What it comes down to is that if you're working with a high headphone jack output impedance relative to your headphones' impedance, and their impedance varies with frequency, the actual voltage delivered is going to vary with frequency as well.
I use HD598s, which are a nominal 50 ohm can - with a jump in impedance to 300 ohm at that 100hz level.
I have gotten away with output impedance as high as 10 ohm, but once a component is up to around 50 ohm, the 598s sound like Dr. Dre has rebalanced the sound. (With an RMS meter, or better yet a USB oscilloscope, and a dummy load it takes about 5 minutes to measure the impedance of that jack. Why more vendors and reviewers don't measure it is beyond me.)
Obviously, the 598 is a more extreme deviation from nominal than the 650, but depending on the impedance of the headphone jack they're plugged into and what music you're listening to, I could see the 650 overemphasizing the low end and that making the high end feel pretty rolled under.
But, play these with a proper headphone amp, and I like them quite a bit. Their timbre matches the timbre of the monitors on my desk very well, and I can play 'em way louder late at night than I can the monitors!