I own the 400. I personally think it does a very good job. I use mine only for aircraft (both VHF & UHF). I've never put a trunked system in the radio to see how it performs. But I did buy a PAR FM notch filter and the change was much better than I imagined... one of the best purchases I've made.
Hopefully I can explain this. A notch filter is a filter designed to deeply attenuate a frequency or range of frequencies in order to eliminate interference in your scanner. The FM filter that I mentioned notches out the FM broadcast band of 88~108 MHz, with the filter centered on 98 MHz.
The filter installs in-line between your antenna and your scanner.
In my case, this filter has far exceeded my expectations. Since I listen to VHF & UHV air, the VHF side is like night and day in the difference. I won't tell you that I hear signals farther away, but the signals I do hear, sound better. As an example of the difference it has made for me. I live approximately 35 miles from Montgomery Regional Airport. While doing a service search on the air band, the scanner stopped on the ATIS frequency... I had never heard this before from home and I now routinely hear the ground side of the tower and approach.
You can find these filters and other filters at Universal Radio, Scanner Master, and Grove Enterprises.
The "problem" is that there are a lot of signals out there and they are not all at the same signal level. Some scanners filter out signals not in the immediate band you are monitoring, but are limited in how well they can work by space and cost.
The biggest problem for me is FM broadcast signals (88 MHz to 108 MHz). When I installed my discone and connected it to an RF meter, FM broadcast was typically 20dB hotter than the "scanner" signals I wanted to monitor.
So I too installed a filter, but not as grand as the one already posted here. I used this filter, which reduced FM broadcast and signals below 25 MHz. It was a good $50 investment and increased my reception immensely.