A couple of thoughts.
- A quick search of Google's Patent section reveals nothing for Phoenix Audio. Patenting is costly and time consuming, I'd be very very surprised if there is a new innovation in that opamp that warranted patenting.
- My understanding is that many of their products are Neve inspired, particularly the later Class A/B era Neve stuff (could be wrong), I'd be comparing what you've found to Neve 340/440 and others like that.
- My understanding (again could be wrong) is that you can copyright a layout but not a circuit
- Unless you plan on selling your clone I don't see how they could do anything to stop you.
I'm on retainer with several pro audio companies and value I.P., I'm certainly not a clone anything and everything kinda guy but one has to be practical. I can understand companies being vocal in this era of cloning.