I must say that the HE-500s have been on my radar for a while. I didn't get on with DT880s very well (I had the 250s), and I really didn't like the K701s I had either. I have been through a fair few mid-fi cans really, and am most happy with D2000s and HD 650s (even though they get less litening time these days).
I'm also in the situation where I only really have mid-fi amps - Shanling PH100 and a heavily modded Xiang Sheng 708B. I find the sound of the 708B suits my needs more now, and I suspect it punches far above its weight in its current trim. Could do with better valves though. I'm hoping that the 708B would be adequate for running high end cans once better valves are fitted.