tl;dr - That's a tricky question to answer, because the answer is
both. Loudness preferences are subjective, strictly speaking, because my loudness preferences may not match the preferences of someone else. Nonetheless, these preferences are objectively quantifiable (and the unit of measurement is watts).
Long answer - plugging my Aeons directly into the RME, with a little bit of EQ to correct the tonality (bump up the bass and tame the upper mids), I sometimes find myself maxing out the volume knob on some songs. One example would be Dire Straits's "Money for Nothing" from the 1985 album
Brothers In Arms - the version in my lossless library is a rip from the original 1985 CD, where the mastering is punchy and not compressed like later remasters. Especially during that intro with Sting's uncredited vocals, I even find myself craving more power.
The SMSL comes in and saves the day. This thing has so much power that I can make myself cringe without even pushing it to its limit, whereas the RME doesn't get that blisteringly loud. Now, my Aeons are pretty inefficient at 83 dB/mW, but there are headphones that are much less sensitive - take the DCA Stealth, for example. Those have a sensitivity rating of 86 dB/mW, and if my RME is struggling to give me as much power as I want sometimes with my Aeons, the Stealth will absolutely present more such circumstances. I bring it up because it's a headphone I'm considering.
Yes, I can disable EQ and the song becomes loud enough, but then I lose tonality. Still other songs which are compressed to oblivion (like the example I posted in
this thread) play loudly enough that the RME provides more than enough power for enjoyment, so I can plug directly into the RME. But why bother being so hands-on all the time? The point of the hobby is to just set up and forget about it, and to enjoy the music with a simple interface - one plug, one volume knob. To that effect, even if the RME works 95% of the time, because the SMSL works 100% of the time, I have incentive to keep it. Plus my desk looks neat and clean, without a ridiculous 2xTRS -> 4-pin XLR adapter sticking out of my DAC, lol - aesthetics matter too.