@jamegla I have some of the gear you are talking about. The Serene and HPA4 are sonically very similar, contrary to some posts online saying that the Serene kills the HPA4. Love both of these preamps (I have the LA4). The LA4/HPA4 has much more useful features. Great thing about these 2 preamps is that they will really let you hear the upstream components (such as a DAC) without editorializing the sound.
When I get a Playback Designs Dream DAC I will use 1 of these 2 preamps. I already have a PDB Plink based streamer.
BTW - Since you mentioned headphones, this Friday I am going to hear what some are saying maybe the world’s best headphone. The new RAAL 1995 Immanis (will also hear the Magna). I have the older RAAL models, which compete with the Focal Utopia and Susvara you mentioned. The new RAAL 1995 phones are said to be much better than what I have.
Today, I received a new headphone cable for the Serene preamp. Unfortunately, the old RAAL phones need more power than the Serene can deliver. The Immanis may work with the Serene and this new cable I got.
A great DAC is really important for headphones.