They will both, 100% guarantee, sound excellent with these IEMs.
The bass quantity and quality will depend more on the tips you select and the firmware that you use than on the hardware in these DAPs.
If you are looking for excellent sound and not the latest model the DX50 will be more than enough.
If money is no concern then just grab the DX90 as it is a bit more advanced in terms of hardware.
The differences in the sound would depend much, much, much more on the firmware and the settings than on the different DACs inside. The DACs and their architecture in the DX90 are considered better in terms of sonic characteristics, still the DACs in both devices are excellent and they are the strongest link in the chain. The difference between the two devices is not that substantial.
And by all means - enjoy whatever DAP you chose!!