Thanks Isaac.
Problem solved I think - the power lead to the tranpsort had almost come unplugged. Tightened up the power lead and the interconnect and all seems to be back to normal again.
The Dac64 does seem to be very sensitive to stuff like this, but it's such an awesome beast I'll live with it. For the record, I don't find mine at all harsh, in my system it's just very very rhythmic, "boppy" and detailed, but I have a "valvelike" (apparently) Bow amp and Sonus Faber speakers so there's not a lot of harshness there to begin with :MILD:
updated 07/11 - b*gger, problem not solved after all, but I spoke to Chord this morning and this appears to be a recognised issue with a free upgrade/modification available. They're having it picked up from my office by courier and it will be couriered back to me, modified and checked over, by the end of the week - 10/10 for customer service!
Cheers, Steve