In your set-up, it comes down to CBA....cost benefits analysis....there is a thing I call "The box" it is a small 2 inch by one inch or so hardware EQ that was open sourced on another forum which is basically like a small hardware equivalent of a parametric equalisers but then you have to weigh up the total cost vs just buying a used pair of 500's and be done with it.It never really gained traction as I have only seen one guy pairing it to lower end Superlux on this forum...the designer says he made it because he always felt parametric EQ introduced artifacts into the background.
There are guys who build "The box" but because they are one-off made for your cans specific sound curve problems, it might be $80+ postage.
Thank you very much for your reply.
I have seen the thread about Superlux and "the box" (some filter, if I am not mistaken), but still it seems a complex and risky path to take.
For me probably the best solution would be to buy HD650 and to pair it with a fairly good amp.
Sorry for asking so many questions about HE-400 and nothing coming out of it.