Hi Tino how can adding distortion( tubes etc) improve the sound ? You are the only person I know of who dislikes the highs on the 5400ES . IMO absolutely no mods required (a total waste of time & money) if using HDMI and SACD MCH. Vince tells me that your dirty mains supply etc. probably your biggest problem
Man, you must be the worst troll on the entire Internet. I have been doing a little research on the Sony 5400 and have seen your clueless posts in three different forums now.
You clearly have no understanding of the things of which you speak. You don't understand electronics or psycho-acoustics, you have a mid-fi system, and by your own admission have never heard this modded player or, I am reasonably sure, not only no Modwright product but no digital player with a true valve output stage anywhere, ever.
Here are a few general topics to research to help you understand why a properly done valve (or discrete solid-state) output stage could definitely sound better than op-amps:
1) The effects of feedback (even high-speed feedback).
2) Parts count & circuit simplicity.
3) Perhaps most important, the sonic effects to humans of the harmonic distortion spectra: low vs. high-order harmonic distortion as well as even vs. odd-order.
That speaks to the output stage. Dan's other mods address things like power supply regulation and jitter - perhaps even you can have some inkling of understanding of the effects of jitter in digital playback.
That's great that your stock player sounds fine to you. Guess what: my stock 5400 sounds great too. And for some people an iPod with earbuds is all they will ever need or want. Good for them (seriously).