Maximum output for the E18 is 280 mW into 32 ohm, isn't it? The HA200/P50 are 160 mW into 32 ohms, IIRC.
If you feel the E18 is cramped, I wouldn't recommend the HA200/P50 either. They're fairly intimate sounding as well. It's also a function of your earphones/headphones. The SE535LTD, while brighter than the normal version, is still an intimate sounding earphone, no matter what. Can't comment on the Sony. It's possible that the E18 just happens to not do well, but I can't really think of why at the moment --- it should be able to handle decently tough loads, given the gain/buffer design that FiiO always uses.
The PCM1792A is a flagship-level DAC when used in the right hands. It can sound exceptional, right up there with the AK4399 and ES9018. However, it's what you do with it that counts, such as jitter control (oscillator, reclocking, etc.), I/V conversion, and analog gain and current buffering. The entire chain contributes. I've heard PCM1792 devices sound terrible (Styleaudio Topaz), and have heard them sound pretty good to great. These days, designers flock to ESS or AKM, but I still think the PCM1792 is a top-tier DAC chip. Again, it's what the designer does with it that counts most.
I will continue to think of sub-$500 DAC/amps, but my brain doesn't work ATM. Do you absolutely need an iPod/Android compatible DAC/amp device? Or is simpler okay?
EDIT: I was going to suggest the Oppo HA-2, but while it's powerful for 16 and 32 ohm loads, I found out that it drops to 30 mW into 300 ohms, which is weaker than the power output of the TEAC/Onkyo at 300 ohms (65 mW).