Not a fan of the DM80, though to be fair I've had some bad experiences with the supplier (decoying uk)... the newer .22 one I had had affected accuracy very badly, and shifted zero about 3 inches left at 50 yards. SAK and Parker Hale are as quiet, just different type of sound - 16" CZ 452 Style. The older style DM80 I have had on my HMR is ok though.
Anyway, back to the HMR - I've spent a great deal of time and money researching different mods for the HMR, as have 2 of my mates - basically there is nothing that will ever make this a quiet round. I also shoot .243, and like the HMR, the most you can hope for is to quieten the muzzle crack so you aren't deafened... nothing will ever reduce the sound of the round going supersonic in mid-flight.
If you listen to an HMR down range, you will hear a pop/crack about a half second AFTER impact, which is the sound of the round reaching you - so your quarry will be dead before it's heard the shot. Its mates will Foxtrot Oscar though!
I personally think the SAK or Sirrocco are the best .17 HMR mods I've tried (in terms of price, size, and noise reduction). I have an older DM80 on mine at the moment, but I'm changing it in for a Wildcat (as soon as they bother to reply to my emails).
General consensus seems to be the PES/MAE Muzzlecan is the actual quietest (but it's big and costs £240). Then the Wildcat Whisper (Still £120+), then all the rest... less than 3 decibels difference between them so humans shouldn't be able to tell the difference.
If I wasn't into wasting money I'd probably just stick with a SAK - which BTW I don't ever clean until accuracy is effected! I spray a light covering of WD40 in it when you first get it, then shoot until it is effecting accuracy. The dirt/coke that builds up helps muffle the muzzle crack and makes it quiter.
IMPORTANT - never leave your moderator on your rifle as it will corode the crown, and never carry the rifle barrel-up as detritus from the moderator could dislodge and fall into the bore... meaning next time you take a shot you could buldge the barrel!