I've finally manage to make some tests with a friend last saturday and all I can say is that the PSB2-1 switch box was really worth buying and very useful as it showed me instantly if there is any audible difference between 2 different headphone amps connected to the same source.
Sorry for this loooong post but I'll try to detail my impressions if it may be of any interest for someone...
For the A/B comparison tests, we've used the following:
Sennheiser HD580 and HD600
Sony CDP-XA2ES as source
Headphones output jack of the above CDP
Beta22 3ch amp
MMM diy amp built by a friend
PSB2-1 switch box with high quality cables to connect to the output 1/4” jacks of the headphone amps
CD: among others: Jordi Savall - Orient Occident | Pink Floyd – Division Bell | B. Springsteen – The Seeger Sessions | Karajan New years concert 1987
I've done the A/B tests with a friend (he built his own MMM and other amps that he unfortunately don't had available for those tests the other day, but he bought the Beta22 from someone else)
For those tests, I couldn't see the equipment and my friend that were behind my back, as he was switching from A to B with the toggle switch without telling me which one I was listening to, and I was telling which are the differences that I noticed. He noted on a paper the amp (A or B) that I was listening to and what was my opinion each time he switches (for example :”more boomy”, “clearer” or “narrower”...).
We repeated the tests with other tracks and other cds.
Then we changed and it was his turn listening as I was switching and taking notes.
The results are very interesting for me, and here is a summary:
A/B comparison test #1: CDP headphones out jack vs. MMM with HD580 and HD600 (I'm not able to hear any difference between the 2, so I mostly used the HD580)
Not big differences between the 2, I noticed one was very slightly more spacious and very slightly less boomy, and at the end we verified that it was the MMM.
My friend didn't notice any difference that he could identify between the 2!
For me it was only in certain parts of a track that I could hear clearly a small difference in spaciousness and boominess , but (FOR ME) the changes are too small to justify spending money on this amp for my HD580, and the CDP headphones out jack is just great for that.
A/B comparison test #2: CDP headphones out jack vs. Beta22 with HD580
Exactly the same impressions as with the MMM. Same conclusions. Surprising as the Beta22 is way more expensive.
A/B comparison test #3: MMM vs. BETA22 with HD580
In this test we were not able to find consistent differences between the 2 amps. We both had the impression that switching from A to B made no difference ! Very surprising for both me and my friend because the Beta22 is known as one of the “best” solid state amp. Well I guess there is no need to spend so much for a Beta22 when used with a headphone such as the HD580 !!
A/B comparison test #4: MMM vs. AUDITOR with HD580
Here the blind test showed that we both notice easily a difference : we both identified one as beeing slightly more aggressive with the highs slightly more present (slightly exagerated to my taste, but very pleasant for my friend). This was the Auditor.
If I were to chose one , I would go for the MMM which seems more natural to me, with no coloration on the high freq. My friend prefers the slightly more high freq presence that comes from the Auditor.
But... for me, all those tests took a surprising turn and everything changed when I made those same tests again but using my friends HD800 instead of my HD580!!
A/B comparison test #5: CDP headphones out jack vs. MMM with HD800
The headphones out jack is immediately identifiable in the blind A/B tests: it sounds immediately more congested, spaciousness is clearly reduced and bass is more “boomy”
M^3: wow what a difference: sound is wider, and deeper, and the instruments are clearly separated from each other. Sound is more “3D”. Bass is less “boomy” and goes lower in frequency.
A/B between these 2 using the HD800 gives us no doubt: we both prefer the MMM.
(the HD800 is really a great headphone)
A/B comparison test #6: CDP headphones out jack vs. BETA22 with HD800
Same conclusions as above. Again, very surprising...
A/B comparison test #7: MMM vs. BETA22 with HD800
Although the sound is way more spacious with better resolution and detail than with the HD580, we can not find any thing that identifies any difference when switching between the 2 amps! So if I were to buy one I would go to the cheaper amp.
A/B comparison test #8: MMM vs. AUDITOR with HD800
same as the test done with the HD 580, but with the highs even more pronounced.
In this test we both easily identify the differences between the 2, and found again that the AUDITOR is more “agressive” and the highs slightly more present. Again, I personnally prefer the MMM for its more natural soundstage, with the Auditor, the sound seems more “colored” to me. This time my friend disagrees and preffered the one which has the voices and high frequences more present, and that's the Auditor.
Final Impressions of this first tests session:
My personal conclusion and advice is that each one has to make his own A/B comparison tests with his own equipment in order to tell what he prefers, and how to obtain a sonic quality that suits him best. At least, my personal experience during this first tests session (I hope to make other tests soon, with the others amps of my friend) , is that the switch box showed me without any doubt that I'll have much better sound quality by changing my HD580 for a HD800, and that I will not really have an improvement if I keep my HD580 and buy an amp... but of course, in this case, it also showed that if I have a HD800 someday, I will benefit to use a headphone amp (such as a MMM for example...).
Now , I'm very curious to compare other amps with the HD800... and I've already started to save money for a HD800...