Toni wrote in message

Well, the case is that I've been playing, among others, with
untuned shielded loops, and they really help reduce interference
by carefully turning them. Unfortunately they also produce very
low signal level, close to the point of not hearing much
difference in background noise when connecting/disconnecting the
antenna. That't why I'd like to gain those extra 5~10 dB by
mathing the receiver to the feedline.

Why don't you just tune the loop? That would cure the level problem,
and also reduce out of band signals. I do agree with the other poster
though. It doesn't take much to provide a usable s/n ratio on HF. If
the noise level increases when you connect the antenna, it should be a
good enough match. MK