What types of antennas are you using? Could be the 652 is getting a better signal...or your 1098 is getting too much, if it's your local system as you say it might be getting overloaded. Try the attenuator. Unplug the antenna while it's RX'ing...the two closest sites to me come in 5 bars with NO antenna when conditions are right.
Also, my 1098, as well as the other GRE800/900 pattern radios I have do not sound as clear as my Pro-197, which is basically the same radio as your 652. No amount of tweaking has solved this, I believe they just don't have as clear of audio or quite as clean of p.1 decode. But from your description of the robot-like voices, I don't think it's that, I think either the signal isn't strong enough or it's too strong. It can be hard to hit a happy medium.
Does your system use LSM? If it does that a whole another can of worms to open up...