I am considering purchasing a used HD400 while I send my HD500 to the shop.  I am wondering what the major differences are and will I be content with the HD400.  My main concern is my use of my HD500 with my acoustic guitar.  I use "vintage preamp", "tube compressor", "reverb", and "ping ping delay" with my acoustic guitar currently.  Does the HD400 have all of those?  Not so much worried about the electric stuff right now.  I have some pretty awesome non-modelled stuff for those.  What the HD500 does for me as an acoustic preamp is insane.  I have had countless comments on the sound of my guitar, most of them would be rightfully attributed to the HD500.  My guitar is OK, but would certainly be lacking without the pod.