Sennheiser HD 6XX Sound much better plugged into an 'amp' but supposedly I don't need one?

Edit: after minor experimentation it seems that the headphones just don't get that loud on the iPhone and I am enjoying the added volume of the scarlet.

Hey phoneheads,

Recently got into the audiophile world after selfhosting Navidrome and wanting to experience the full glory of my 24bit FLAC files. Grabbed a pair of HD 6XX off marketplace and have been really enjoying them. I experimented by plugging them into my Focusrite Scarlet 2i2 as an 'amp' and experienced a notable improvement in sound. Obviously they were able to get louder but it also felt like they had more substantial lows. Most people say the iPhone dongle is all you need but now I notice a flatness when listening on mobile vs my desk setup. Am I experiencing placebo or what's going on here? Please advise.