Hi Scott, thanks for your feedback.
Was the droning caused by your aftermarket back boxes only at motorway speeds or was it evident at other times as well? My car is purely a weekend fun toy so if it's only really an issue for high speed cruising it's not so much of a problem for me.
I reckon the reason why most aftermarket systems end up having much smaller mufflers than stock is the unusual arrangement with the R171 where the inlet is on a 45 degree angle right at the back corner of the box. It's pretty hard to find off the shelf sports mufflers with that kind of setup.
bstevens52 has said he's pleased with the Magnaflows on his car so I'm thinking if I stick with them I should be OK, unless someone can tell me otherwise...
The main time the drone is intrusive is at motorway speeds. However, at lower speeds it is still there any time you touch the throttle, and just gets very tiring after a while.
My old 320 was quiet inside the car until you put your foot down, but was not intrusive, just a great exhaust note.
I think you are probably right about the size of the boxes vs angle of the pipes. The boxes I have are much smaller than stock, but fill the space diagonally, but have loads of space around them
As has been said in many posts, sound/ noise is very subjective.
I too only use the car at weekends, but want it to be civilised and luxurious when cruising, but loud and a great sound when you put your foot down, hence my leaning toward the AMG boxes.
I cannot comment on the Magnaflows, but if you can get to listen to them first, I think it would be worthwhile.