Keep the 706 for VHF/UHF communications and as a backup for HF.
When you seriously think about a second HF radio, look at the TS-590 or 756PRO. By then they should be in your budget range. Both good performers for casual operating.
The best improvement you can make is in the antenna system.
I'm looking at the TS-590 as a distinct possibility. I've never owned a Kenwood, but they get great reviews on here. Thanks.