I have this same setup + HMS and WAVE Storm's. To me there is zero need for an amp. Plus above 0 it is LOUD. I just finished listening to the remaster of Pigs on Animals and the middle section bass goes so low my teeth rattle. Many on this thread def have "audio-itis" and can't leave well enough alone. If you've ever owned a major bucks big rig think Shun Mook "Mpingo Disc" or a nice $10,000 power cord (for preamp, DAC, upsampler, two monoblocks). If you want some tube sound get the Feliks Envy. I know a few on the Forum who own these and are quite happy. Also the MSB amp looks promising.
As long as you're happy, that's all that matters. But it's not about volume. The Dave can drive any headphone to audible levels, it's about the quality of amplification. Even for low impedance headphones, there's a clear improvement with a quality headphone amp. I get choosing to not spend money for that difference or being content, but also totally get people wanting to maximize amplification for headphones they've invested $4k in.