I'm not a big coin hunter but here is what I'vefound to be true for me and relics.
I've been detecting since 1992 and in my honest opinion, the only thing I seem to find more of in wet soil is...IRON. The iron signals that I don't hear so well in avg to dry soil sound "good" in the wet soil.
I haven't particularly noticed deeper signals in wet ground, just more of them.
Avg moist ground is where I get the greatest depth, but dry-dry, hard as a brick soil seems to boost shallow targets, just harder to diggum.
Having been a dealer, I have used some of most brand machines. I currently use a Nautilus DMC-2b and have a White's Blue & Gray and a Fisher 1266xb as backups.
Had a lot of success finding frags with a DFX & Tejon and my only cannonball (CS 6#bormann) with the Tejon.
Go figure.
Burt