ATT/overload
The PSR500/600's are notable for having issues with overloading. Most users notice this on the VHF-HI band (138MHz to 174MHz) and the civil air band (108MHz to 138MHz) as you have. But it also occurs on the other bands. Where you have the most problem is a subject of where you are in terms of the local RF environment.
In an ideal (no overloading interferers) situation, most seem to agree that the receiver on the PSR500/600 is quite sensitive. But, as you have said, this becomes useless when it is overloaded and the attenuator must be used. It is not true, as some believe, that this is unavoidable if you want high sensitivity - but to avoid it it does become increasingly difficult, in terms of the design, as you increase sensitivity (and tunable frequency range). In professional grade receivers you can throw money at the problem and solve it with good design and high quality components. These are luxuries that may not be so easily tolerated in consumer grade price class equipment. Hence the compromise. Sensitivity is actually an overrated spec for scanners except in very rural areas. So many more RF design points really need to and should be emphasized. Things like image and 1/2IF rejection, intermodulation, and dynamic range really need to be designed for with gusto! But, for many who are not in the field, they only "see" the sensitivity figure as a go/no-go selling point. Sensitivity, by itself, is probably one of the easiest things to "optimize" IF you don't consider anything else! But USABLE sensitivity in a non-ideal environment is a very different matter!
Based on what I have read, I believe that Uniden decided to sacrifice some small amount of sensitivity to offset the necessary compromises in the receiver's RF design. Actually, though, you can rephrase that to say that they simply set the sensitivity at a USABLE point relative to the rest of the design given a set RF environment which included some amount of likely interference while, it seems, GRE may have ramped up the sensitivity and designed for a much lighter level of tolerable interference. That may account for a little of the difference you and others note but there are far more elements to consider. Most receivers used today are based on the tried and true superheterodyne design. Broadly put, the idea is to convert a wide range of received signals down to one relatively low (sometimes high, but usually low) "intermediate frequency" or "IF". Therefor, you need only design the rest of the receiver's elements to deal with that one IF. This is a nice design but it has its own problems. The conversion of the received signals to that IF takes several steps. During this process things can and do go wrong. The wider the range you want to deal with, in terms of receivable frequencies, the worse the potential problems encountered. A cell phone can be made small and relatively simple because it covers a relatively small subset of frequencies compared to a wide range communications receiver or scanner in addition to the fact that a typical cell phone is designed to work at very high levels of desired signals (the interference is just "swamped" by network design). A professional grade handheld two-way radio also is typically designed to cover a relatively small range of frequencies compared to a typical scanner (there are, of course, some exceptions) and, again, much more money can be thrown at the design. In any case, each manufacturer chooses the "path" their receivers take in terms of how the RF gets converted to the IF. These "path" choices are referred to, in RF engineering terms, as the receivers' "frequency plan". Each manufacturer may choose a different "plan" and may choose different "plans" for different models. This is why you will find some users who find that brand X works well in one high level RF environment (like a given metropolitan city) while brand Y does not yet in a completely different high RF environment (different city) just the opposite seems to be the case. Depending on where you are and what strong high level RF signals are present at your listening location one brand or model may work fine while another is nearly useless - all because of the particular problem signals causing issues with the designed "frequency plan" in one design but not in the other. It's really all about the "numbers" - "numbers" on one "path" (between RF and IF) have issues in location X but not in location Y while numbers on the other "path" have issues in location Y and not in location X. Of course component quality and design quality also come into play. It's a little like one person being forced to drive one way to get to a destination and dealing with traffic problems heaviest during times when school traffic is high while a second person is forced to drive a different way and dealing with traffic problems heaviest during rush hour. As long as the school-path dude drives during non-school traffic times he's fine and as long as the rush hour path dude drives during non-rush hour traffic he is also fine. But increase the school traffic and, while the rush hour path dude is moving along nicely on his path, the school traffic path dude is not so happy. Then when the rush hour traffic starts - the situation is reversed. In both cases, traffic could be said to be "high" but, because of the different "paths" used, each driver gets adversely affected at different times. Of course, there will be cases occasionally wherein the school traffic and rush hour traffic may be high simultaneously in which cases only the helicopter, airplane, and hot air balloon dudes are doing ok (weather permitting, of course). And, the quality of the roads and the cars each driver is using also come into play. Now, replace time with radio frequency signal level content, the driving path with the frequency conversion plan inside the (two to be compared) radios, and the destination with the IF that all the RF must be converted to and you may get a rough idea of what I am getting at (helicopters, airplanes, hot air ballons => professional grade receivers in this analogy, BTW). Different "paths" will yield different characteristics in any given environment, even when using same quality components.
In your case, it is extremely likely that some high level signal, likely within the 80MHz to 200MHz range, is swamping the PSR500's front end and causing your VHF-HI reception problems. Given it seems to always be there, I would look at either a FM broadcast station (88MHz to 108MHz) or a television broadcast station (channels 7 through 13).
Because of the design of the Uniden's "frequency plan" and, very possibly, some better front end filter usage and/or design, it is not suffering the same issues.
You should first try and find the cause - again, FM and TV being the most likely culprits, followed by pager transmitters and taxi base stations (based on my experience) as they broadcast "almost" continuously and at very high levels (especially pager transmitters). Once you find the problem - then you could possibly purchase a notch filter which can tune out the offending signal. Of course, if there are are more than one such interferer then it will be more difficult.
-Mike
