Is there any question? 
Is there any question? 
aiacuone, one thing you never mentioned is what type of music you listen to.
would you say its worth getting the amp?
I doubt an amp is going to change your opinion toward the HE-400. Save your money and find another headphone.
i listen to all music, but if i had to narrow it down..... id say more leaning towards 70s rock along with house/electro.
When i bought these i guess i was expecting this endless perfect deep profound bass when i chucked on a electro but also had clarity with the rock.
I dont think im going to get a very good balance with these cans. I dont to be able to get that massive thump from a drum kick but also get the gradual deep beats of a rnb song. The problem ive got at the moment is that ive adjusted my levels for everything pretty well, the i put something like this on (please dont attack the song... old favourite, love the bass line) http://www.youtube.com/watch?v=GsyJVv-9ygM, the cans just distort and i find myself either turning down or adjusting it.
In the back of my mind of got the idea that an amp would help that but theres also an element of doubt.
Well, that confirms it then.
I think so. Something like the E17 is nice to have, no matter what headphones you use. For the HE-400 specifically, it honestly doesn't sound that much different to me from the E17 (EQ aside) and straight from my laptop. But I have a Triple.fi IEM where the difference is huge and very noticeable. That really surprised me.
Maybe you're a basshead in disguise. I personally don't buy all the amp talk. If your macbookpro is a newer model, there's no reason why its hp out shouldn't be bad at all, in fact it should be quite good.
Law of diminishing returns, alot of these orthos almost have a ruler flat response from 1 or 2khz to 20hz and treble rolloff after 2khz. This is good for classical music but can leave electronic music sounding too harsh and lacking bass.
Well the problem is they shouldn't be lacking bass at all compared to the 555/595. Those should sound like tin cans in comparison. But there is nothing too exaggerated about the he-400 bass. It still is overall relatively neutral.
i think so brother. Ive been flicking through a few songs and ive come across some metallica songs. Metallica S&M has some good audio quality and tweaking the levels to this album in particular is paying off. I cant remember who typed it, but someone suggested to lower the levels of everything else for the lower frequencies to take over a bit more. The bass has a brilliant sound to it but its been hiding behind the higher frequencies.
I guess its all part of it hu? Ah well, back to the drawing board and more adjusting
Compring your 555's to the HE400
http://graphs.headphone.com/graphCompare.php?graphType=0&graphID[]=3651&graphID[]=563
Regarding the HE400
They look pretty flat from 400 to 20hz, things get a bit messy from 1khz on. A 10khz spike is pretty common "voicing".
Please do not compare those two graphs and say the Senns have more bass to 30hz. That's the last thing you should do. The two grahps are not even level matched.
Here's a better representation of the two compared to each other (inner fidelity graphs)

Sorry but this is quite incorrect. The bass produced with the he400 on velours does not change from pleathers. I know that it sounds contradictory but it's true. Feel free to ask around of those using the velours.