Hi Ekul61,
I'm not sure what amps you are looking at or what your budget is. But I will try to answer your questions. I just updated my kit from a Asgard 2 to a Oppo HA-1 and a Little Dot MK VI+. I like to play with tubes and SS amps at the same time. Things that changed in this upgrade are great punch, more power, wider sound stage, greater dynamics. I'm sure there are more differences but I've only had these for a few days. Overall it is a much more enjoyable experience. The HE-400, even though they are on the efficient side of orthos, love all the power they can get. The 5 watts that the Little Dot MK VI+ puts out really makes them sing! I think that the biggest difference I notice is using a balanced headphone output. The SE output is a lot better than the Asgard 2 on both units I have but the biggest gain is from the balanced output.
One amp that you may want to upgrade to is one of the Audio-GD amps. I've heard really good things about them. They are affordable and put out a lot of power, which your HE-400 will love! The NFB-6 looks really good and comes with balanced and SE outputs:
http://www.audio-gd.com/Pro/Headphoneamp/NFB6/NFB6EN.htm
My 2 cents
Jeff
PS: One thing I forgot to mention. The Little Dot really tames the frequency spikes that the HE-400 exhibits too. If this bothers you you may want to look at the Little Dot. A bit on the higher cost side, but I really think it is a very good match with the HE-400.