With the PS15 I noted about 10 to 20% added volume (I could roll back the volume by about that much to get back to the db I was used to). The LED seems to be burning brighter and the tube seems to be burning brighter as well, as noted by desertblues above. I did not see a great amount of difference in the quality of the sound on the HE-400's via the PS15. I would add that since I just changed out the power supply today my experience is limited. I plan to swap the PS15 and stock supply out a few more times to see if the audio quality is that noticably different over the next few weeks. The HE-400"s are easily driven so the Bellari is more than adequate as a headphone amp.
I generally use two setups 1) at work MBAir>Bitperfect/iTunes>Dragonfly DAC>Senns 595
2) At home MBAir>Bitperfect/iTunes>Dragonfly DAC>Bellari HA540>HE-400
I have tried the HE-400's and Senns with and without the Bellari. The Bellari makes such a big difference that I have thought of getting a second one for the office. The refinement in sound however distracts me to the point that I can't get as much work done.
The HE-400's are the best headphones I've ever owned. They seem to be able to separate sound bilaterally as well as around each ear cup. When you hear rain on Uncle Albert/Admiral Halsey off the album Ram (Paul McCartney), you sonically feel like the rain is falling just outside your ears. You can literally hear individual raindrops coming down. They seem to be able to make sound 3D, expanding the soundstage while significantly expanding bass and sub bass. I have heard Uncle Albert many times over the years but now I can hear sounds I have never heard before. The Senns 595 seem flat in comparison to these. For the price point you can't go wrong. I can't compare these to the upper end HiFiMan and Audeze headphones. I wanted to see what Ortho technology was all about and at about 1/2 to 1/3 the price I feel like it was the best headphone purchase I ever made.
With the Bellari the sound coming from the tubes seem to make the biggest difference in sound quality. As stated before the Telefunken NOS were 2X's sonically better than the JJ gold pins and 4X's better than the stock Ruby's.
One other thing I would mention about the HE-400's the co. tried to further refine these from the 2nd version (rev 2) to a rev 3 which did not work out. They have since pulled the bad versions and as noted in the HE-400 thread the new version appears to be fine. There are plenty of pictures on that thread to identify the recalled versions.