I was looking for a preamp for my Gustard R26 dac, and was torn between the H26 and P26.
I chose the P26 preamp based on some (really helpful) advice from the Gustard support. The choice was driven by the following aspects:
1) I did not need the headphone amp, just a pure preamplifier.
2) I have swapped the LME49860 op amps on the P26 with Sparkos Labs SS2590. The Sparkos pair cost me 250 Euros, which took the P26 to a whole new sound level, that is hard to describe. The LME49720 op amps on the H26 are also on swappable DIP8 switches. However it has capacitors mounted very close to the Op-amps, and the Sparkos OpAmps are big and need space (LxB is 1.1 x 1.1 inch, or 2.8 x 2.8 cm). Gustard was kind enough to send me close up measurements that helped me conclude that the op amp swap would be tricky, or just not possible.
3) The P26 allows you to adjust gain between 0 to +6dB both in the input and output stage, which helps to harmonise and match with components on both ends. The H26 only has +6dB gain option at the output side.
4) Gustard support responded to my question of how both would compare in pre-amp mode with the following reply, I quote: "The P26 is a good choice for a preamp only. The P26 and H26 preamplifiers have different styles, the H26 is tuned a little softer while the P26 is a little clearer & transparent."
All in all, a good choice, as I said..