I’ve played the same tracks on different drivers and reached the same conclusion. Take a look at my Utopia review on this site – especially row 4 of the comparison table at the end. Per extensive back-to-back comparisons, I heard only two levels of volume (limited dynamic range) from the HD-600 but much more dynamic range from Focal’s products. This may result from driver materials and construction (e.g., plastic dampening vs. precise metal).
Before I wrote that review I bought a Schiit Magni 3 (not the 3+) for my HD-600. To my ears the amp simulated recording compression with every complex, noisy, and distorted rock track I tried. The Magni 3 had terrible treble spray to my ears, so every time it was supposed to go loud my perception went upside down – it sounded softer during loud passages. That was purely from the amp.
Sample test tracks: early Pixies albums, Nirvana’s Smell’s Like Teen Spirit, Sleigh Bells Treats, etc.