Yes, I am 99% certain it is the headphone amplifier. The HD650 headphones claim a frequency response of 10 Hz–39.5 kHz, ±10 dB, which seems sort of unreal and superhuman for a set of headphones, but we’ll accept it for now. The real number we’re concerned with, however, is the HD650’s 300 ohms of impedance. As with other Sennheiser open-air headphones, they have a high impedance that requires not only a powerful amp, but a quality amp. To get the most out of these headphones, you’ll need something good.
I have the Sennheiser HD-600 which I like very much. But it will sound terrible if I just plug it into a receiver or pre-amp headphone jack. The Rotel RC-1070 uses a low power op-amp IC for the headphone jack which is totally inadequate for the HD-650. A good solid state headphone amp can be had for well under $200. I will not recommend the OTL tube amp for this headphone. Usually, the quality is not there for inexpensive tube headamp.
I am about your age. Yes, our hearing deteriorates over age. My hearing starts to drop off at above 8,000 Hz and disappear completely above 12,000 Hz. It does not hinder me from enjoying music. You can check the hearing yourself easily using an equipment (speaker) test CD such as the one put out by Stereophile Magazine.