Question for the group. The OP is looking for "slight bit of warmth, plus better dynamics and imaging" and is asking for pre-amp suggestions to help with that. Is a pre-amp the right tool to solve this issue? Or would looking at different headphones make more sense?
It (somewhat) depends who you ask. This approach is not without precedence. The traditional "problem" with Stax headphones specifically, compared to dynamics and planars, has been lack of warmth (on some models, like 009) and weight in bass. There's a guy who was using a high-end tube preamp into a $14K Eksonic T2 (direct drive electrostatic amp) to try and "flesh out" the sound. But after reporting initial success, I'm not sure he stuck with that setup. Another example: the Woo 3ES is reportedly a fancy 300B preamp stage bolted in before an Apex op-amp based direct drive circuit. The implication being that the 300B stage in this case is not really necessary; it's there to color the sound in a pleasing manner.
In my somewhat limited experience on the matter of pre-amping a headphone amp - you can move the needle a bit in the intended direction with a preamp, but indeed I would lean to the camp that one should keep searching to choose a headphone + amp combo that is more attuned to your preferences, rather than try to add another preamp upstream. I found my "match" with a different T2 build (different HV transistors makes a HUGE difference, more than rolling the tubes in these amps) and Stax 009 headphones :)