I have one 800 simulcast county just south of me that can't be clearly monitored on anything but a x36 scanner set to MANUAL 10. That works perfect ANYWHERE in that county, as well as my home area out of the county. My TRX-2 DSP level can't be adjusted to work the same. The problem, as I see it, is the "AUTO" algorithm. When you have phase distortion (due to multipath), or weak signal, these AUTO sensing adjustments go whacky, and there is where dropouts occur. The MANUAL adjustment on the Uniden x36 (exclusive) allows you to center the modulation peaks on an optimal fixed center...thereby eliminating any "AUTO" drift as it attempts to hunt for a better setting. Are there some systems that are so out of phase that even MANUAL threshold settings don't work? Some people do report that, although I have not encountered one in my travels yet.
After watching the AUTO threshold BER measurements work on my 536, and doing extensive weak signal testing down to ZERO bars of signal, I have concluded MANUAL 9 was optimal for VHF and most UHF systems I listened to. 700 and 800 systems leaned towards MANUAL 10 for lowest BER. I will say some individual 800 transmitters in my county were out of this spec, and those same radios reproduced the same results every transmission, so those radios must be out of spec. I work in the industry, so trust me...it happens.
As for my testing with my WS1080, PSR-800 and TRX-2, the best DSP setting I was able to "sort of" see any weak signal P25 improvement with on distant VHF signals was 74. And I did go further above and below the default and noticed more "Donald-Ducking" given a fair amount of signal as I got away from 74. Did I have a big difference at 73 or 75? No, but after testing the more severe dropoff limits above and below 74, it ended up centering on 74. As for 800, that value also seemed to work, but nothing worked for LSM issues. And all my tests, accept final mobile tests, were done on a fixed outdoor Diamond Discone antenna with FM broadcast filtering in place. I compare all my scanners side by side using a Stridsberg multicoupler. Very stable test platform.
Phil