You should imho first stop using the stupid stealth meter, then re-think your tactics (showing little profile for instance, going down to 1 knt instead of 2, etc..) U-boats were there to sink merchant shipping at sea, not to troll around inside british harbours...
PS : re the ASDIC you don't hear, the DDs have also good hydrophones. You can at night get closer running surfaced than submerged if you are careful with the profile you show and not speeding.