I should add that to recover from selling the HPA4 preamp, I have ordered the $500 Schiit Valhalla 3 OTL/OCL headphone amp/preamp. It only takes 1 RCA input and has no remote. However, I am getting an audiophile level custom RCA switch built by SpaceTechLab. They were recommended on A’gon.
This passive switch with a knob for input selection (not degrading audio quality) will go into the Valhalla 3 single RCA input. The RCA output of the Valhalla will go into the CODA #16 RCA input. I will also have my tuners connected by RCA (only way) to the passive switch.
I will also have the Byggy connected directly into the #16 with the XLR
Volume for both RCA and XLR can be done via the iPhone, and the Valhalla 3 will have a volume knob for the RCA path, just like the AsgardX.
The reason I am trying this is the best preamp I ever heard is the $33k Allnic OTL/OCL top end model. I am attempting a poor man’s equivalent. Here is a quote by Schiit.
Nothing Between You and the Tubes
With Valhalla 3, there’s nothing between the tubes and your headphones. No transformer. No capacitors. You’re hearing nothing but the tubes. This OTL/OCL topology makes Valhalla 3 the rarest of breeds, an amp where tubes directly connect to your headphones.

