I have not played both but I have borrowed a friend’s Abyss a few times, matching it with a couple of different class D power amps, and I even used it on two recordings. It’s an amazing piece of gear, I’m a big fan of it. My opinion is that if you are into this kind of amp, which of course comes from a pretty recognizable lineage of bass sounds, the sound of this pre is timeless. Which means, at least for me, that I could buy a Zephyr and keep it for the next 20 years without any interest in looking for an ‘upgrade’ or an ‘update’ of that kind of sound because I don’t think that there is one (I must also say that I play through a lot of amps for work, like many bassists do, but unlike many I’m not always itching to upgrade my gear. That’s just me, I’m not judging negatively who does otherwise).
On the other hand, in my personal opinion/perception of the trends in the bass amp world of today, powers amps are more susceptible to change and/or update at the present time and I think many of us can see room for improvement in juggling tone and practicality/weight.
So, my response to your question would be that you should get the preamp - because that to me is the timeless part of the sound - and match it with whatever seems like a valid power section that the market offers today, class D or otherwise, and update that when/if better products come along.
