Barry, I have the dual Sports Librand fitted to my Catted 1999 Plus 8 and it's fairly noisy and suits me fine but as my wife is not a Morgan fan we don't tour in it at all and my runs are within the county in general.
On the occasions that I have taken it across country on the A and Motorways it really is too much of a droning noise to be honest with you and my hearing is poor, good luck with your choice, great cars these enjoy.
My first Plus 8 was a single sided standard exhaust and as you say is a whispering sound.
