To be honest, I was never a huge fan of the HD650; it always sounded rather sluggish and dull compared to the 600 or even the HD800s.
Neither tube amps nor ifi’s ZEN stack convinced me when paired together.
Until, yes, until I received the Nitsch Pietus Maximus.
Together with the Holo Cyan2, the 650 is now more or less my daily driver.
Since the Nitsch was apparently tuned specifically for the 600 series, these headphones are unrecognizable since. Dynamic, fast, relatively punchy bass and clearer in the higher frequencies without losing the valued qualities of the mids.
Incidentally, Nitsch offers a tuned 600 version specifically for this amp for just under $1,000, which I don’t think is necessary 