I originally thought that these passthru things were similar to splitters, ie pretty much a direct connection to the slave receiver except for DC block in at least one direction. However this does not seem to be the case. I think most of these tuners have some degree of amplification, and they are not at all "a
straight through connection".
Another thing that is obvious, is that different tuners are very different. The passthrus in some tuners seem to not only pass RF when the master is turned off, and some people report even passing the DC to power LNBs, while the passthru's in other tuners seem to go completely dead when the master is turned off.
Re to quality improving through the passthru, I have seen both better and worse through a passthru. I have also seen situations where one receiver will improve and another receiver will get worse when connected to the same passthru.
It seems logical that quality would always get worse with each step down the line when using passthrus, and that IS what I've normally observed, however in the cases where I've observed the quality improving, my suspicion (ie wild guess) is that the power supply in the slave receiver may be of poor quality, and when it's forced to actually power the LNB, that the signal from the LNB is of worse quality vs when the master receiver powers the LNB, if the master has a cleaner power supply, it might mean that better signal is coming from the LNB. In some cases, it could even be that the slave power supply isn't even good enough to provide adequate power to the LNB.
Anyway, although signal filtering might be involved, or even the master being somehow a preferred path for the noise due to differing impedances, or perhaps some form of the FM capture effect coupled with the amplification making the real signal predominate........... I really think that this is a power supply issue.
