The 800Mhz freqs are definitely being used right now by the system.
I am not sure what the story is behind a primarily 700Mhz system having a few 800Mhz freqs thrown into the mix.
Were you on foot or in a vehicle? If you were in a vehicle, your issue with the garbled/distorted audio could be that your antenna is under the metal roof of the vehicle. This has always effected signal strength but has really become evident with P25 systems, in my experience anyway.
I have looked at mobile antenna for attaching to my vehicle specifically for this system alone. Have not bought anything simply because I do not spend enough time in my vehicle with the scanner to make it worth my effort. Not outrageously expensive if you were to go that route.
Depends on if you are going to go with a 700-800Mhz antenna or something for a wider listening range, what kind of mount you choose, etc. I looked at a Unity Gain antenna for the car that was only 3.3 inches in height plus height of the mount. Right around $20.
Dedicated 700Mhz Mobile antenna,
700 MHz Range
Wider range of Mobile antenna,
Mobile Antennas
Just have to watch that you match the antenna you choose with the right style of mount and make sure you have a large enough ground plane if the antenna you choose requires one.