Hey guys, first post here! Anyway, I managed to get a friend of mine to lend his pair of HE-400s to me for a while, and I've grown to really love the sound (coming from an Alessandro MS1). Only issue is, I can't help but feel like I'm not anywhere close to its full potential with my current amp, which is a FiiO E10. 
 
My budget is roughly 700-800 (with room for a bit of flexibility in either direction). This is where I'm presented with a dilemma: Do I invest in a good amp to pair with the HE-400 (like the Schiit Lyr or O2+DAC) or would it be better for me to take that combined cost and just straight up buy the HE-500 instead (which means sticking with the E10)? Ultimately, which option is more worth it?
 
I understand that this is a somewhat specific scenario, but if anyone has a general opinion on similar situations, feel free to say so! Thanks for the help!

I think the HE-400 and HE-500 deserve a much better DAC/amp than the E10, and definitely need more headphone power.

I would go for the Modi/Magni combo over the ODAC/O2. It's rated to have almost double the power of the O2 at 32 ohms, which is a few extra db of headroom. And it's cheaper than the ODAC/O2, but considered by most to be equivalent in SQ.

If you liked the HE-400s, there's no guarantee you will like the HE-500s better. They have slightly different sound signatures, and some people do prefer the 400s: http://www.soundandvisionmag.com/blog/2012/02/08/review-hifiman-he-400-and-he-500-headphones

cel4145 said:

I think the HE-400 and HE-500 deserve a much better DAC/amp than the E10, and definitely need more headphone power.

I would go for the Modi/Magni combo over the ODAC/O2. It's rated to have almost double the power of the O2 at 32 ohms, which is a few extra db of headroom. And it's cheaper than the ODAC/O2, but considered by most to be equivalent in SQ.

If you liked the HE-400s, there's no guarantee you will like the HE-500s better. They have slightly different sound signatures, and some people do prefer the 400s: http://www.soundandvisionmag.com/blog/2012/02/08/review-hifiman-he-400-and-he-500-headphones

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I second this opinion.  I have both the he-400 and he-500, and use the M&M stack.  It seems to have more than enough power to drive both headphones.  I have found that the difference between the headphones is really dependent on what music you are playing though, i.e., with some tracks the differences weren't as pronounced while other tracks I played it was night and day.  However, the he-500 is not an upgraded he-400, it is just a different headphone with a different sound signature.
 
I really don't think you could go wrong with either, and if you are enjoying the he-400, grab the M&M (or ODAC/O2) and, well, enjoy.  Being that Hifiman allows returns within 30 days with no restocking fee, you may well be able to compare them and see which you like best.