It's not so much the resistance of the Abyss as its sensitivity - 88 dB/mW (more sensitive than the HE-6 at 83.5 dB/mW, but still amongst the least sensitive production headphones). 120w/channel into 8ohm will significantly more power than is needed - the main thing will be volume control, as you will not have much play on the volume between silent and way too loud, so you'll want a good volume control with enough fine gradations to dial it in. With my HE-6 when I've connected to speaker amps I usually connect the speaker taps adapter with amp off, zero volume on amp, power on, double check volume is at zero, plug in headphones, play music and cautiously increase volume until at correct listening volumes (to disconnect zero volume, unplug headphones). I'd feel perfectly happy doing the same with the Abyss (I don't need to though, I've heard it on the CMA800R monoblocks that I currently use for my HE-6 and I won't be chasing other amplification for a very long time), but it's certainly a bit nerve racking at first until you're accustomed to it and not actively anxious about destroying your expensive headphones. If you're not careful enough then there is a real risk of damaging the drivers, and they would not be eligible for repair under warranty (and this is entirely reasonable to me, this is most definitely not a use the manufacturer intended). End of the day it's your headphones, your money, your risk. If it all goes wrong there is nobody to cry to, only you can decide if you want to try it (EDIT: Sorry if that comes across as harsh, I just want be clear about the risks rather than sugar coating anything or offering glib reassurances).
You may wish to check HE-6 and speaker amps for headphones posts to learn more.
