Long story but my ABM didnt have an HPF, so Ashdown sent me one (for free!) They had a spare fully populated board, from an ABM1000 I think, so I'm going to get it fitted. That way it's all inside the head and is custom designed for the amp.
Cant beat Ashdown for service! I would stress that although I'm hugely appreciative, I wouldn't expect they'd do it again as I think it was the last spare board in the workshop.
So, still feel the 'need' for an HPF but I've had a change in amp circumstances, rather than a new amp really!