You may want to look at the signal routing and the leveling loops. I don't know the 83592, but typically a lot of different activity will be involved with making the low-band signal - the LO and downconverter as you have been discussing, rearranging some RF paths (including mechanical relays, which can get noisy), different power level detection loop, and associated interconnect cabling.
You may be able to see a noticeable difference by changing the band between 0 and 1 lots of times - like maybe a hundred, with power out set to maximum, to exercise the function switches and relays. If that makes no difference at all, then there's probably a circuit problem in one of the above areas. Also check all the RF connections to be sure they're correctly tightened.
Ed
