Let’s see …
For sure, all of the amps we’re taking about here warrant being fed by something more capable than the Modi Multibit (which is a lovely little DAC, but it’s no ADI-2 DAC or Gungnir etc).
If you’re considering the RME ADI-2 DAC, you might want to give it a try feeding the Clear directly from that before you spend the money on amplifier. An external amplifier of the level we’re talking about will definitely be better, but you may find the native output in the Clear to be as good as you need/want (they’re very easy to drive and due to their very dynamic nature don’t seem to be as affected by the slight dynamic compression the ADI-2 DAC’s native headphone output can exhibit).
For me, I would probably lean towards the RME unit as a DAC to feed the Phonitor X.
I would remove the Loki from the chain and use the RME’s EQ as it is more precise, despite being digital in nature.
And if you wanted to have some tube-flavor from time to time, then as long as the DAC you choose has dual outputs, you could do something like run the XLR outs from it to the XLR inputs on the Phonitor, and then run the RCA outs via something like the iFi iTube 2 and back into the amp. Switching inputs on the amp would then let you use the different tube options on the iTube 2.
I’ve heard the HPA-1, very briefly, but have not done a direct comparison between it and the Phonitor X. Using a Chord DAVE as the source, and comparing to its direct headphone output, the SPL unit is, so far, the only one I felt wasn’t losing anything and was simply bringing enough grunt to the table to drive headphones that DAVE isn’t quite powerful enough to get the best from.
It’s entirely possible with more time with an HPA-1 I would find the same is true there, but that wasn’t the immediate impression I had when I, again briefly, got to try one. And to be fair the situation wasn’t one in which that was a test I really could make with any degree of reliability. That’s something I’d definitely be game to try again properly, given the opportunity. But at least for me, the $1,000 price difference, and the lack of a few features I need in a solid-state headphone amplifier, I’m likely to stick with the SPL unit unless the HPA-1 was discernible (as an improvement) in a blind(ish) comparison.