It used the RKBatchtool. This should be the short steps I did.
1. Download the RKbatchtool http://www.freak-tab.de/freaktab/Rockchip_Batch_Tool_v1.7.rar
2. download the latest 1.5 FW with CWM recovery and the latest Rockbox.zip file. (You can get that from the first page of this thread.)
3. Use the RKbatcktool to flash the update.img file from the 1.5.0 FW with CWM recovery.
4. Boot the normal iBasso FW and copy the rockbox.zip file to DX90 internal storage.
5. Boot into CWM recovery mode and install the rockbox.zip file.
I guess what I'm asking is how do you utilize the RKbatchtool to flash the update? If I read this correctly there are 3 files to download, not just two?