Yeah, they’re not affected by output impedance, BUT the quality and power of the amp make a big difference.
While a T50RP can reach enough volume on an iPhone 6S (at almost max setting, ha!), the soundstage is greatly compressed, bass is less impactful, and generally it will have a weaker sound compared to a desktop amp. And the T50RP is super hard to drive, lower volume output at any given setting than any other headphone I’ve tried yet, but it’s 52 Ω impedance is also really low. Meanwhile, my Sennheiser HD 58X Jubilee (150 Ω) get plenty loud on almost anything I plug it into (including a tiny DAP I have that’s about the size of the square iPod Nano, a bit thicker), but it really sounds special from my higher end sources and amps.