So revisiting the impedance question, A30 outputs:
3.5mm = 30ohm, 6.35mm = 10ohm. Looking at my headphones, I was under the assumption that full sized headphones would almost automatically sound better on the 6.35mm than the 3.5mm given the increased power of the 6.35. Especially headphones which aren't that sensitive and require to use the gain switch because the +0dB isn't loud enough at max level, but looking at the specs of my headphones, it seems that basically all of them except the 990 would benefit from being plugged to the 3.5mm output!
SR80 - 32 Ohm
K550 32 ohms
400i - 35 Ohms
598 - 50 Ω
Q701 - 62 ohms
DT990 - 600 ohms
From the manual:
So yeah my quick impressions was that the 3.5mm sounded a tad bit smoother on the 400i, and I guess that it's supported by the specs & manual recommendations... So if you have a Topping, might seem strange but yeah 6.35 to 3.5mm adapter might give you a subtle/marginal improvement! Oh, and voltage switch set to Lo instead of high!
It's also interesting to note that different headphones react differently to the amp's output impedance. For instance, as mentioned the 400i sounds a tad bit smoother / less harsh (subtle enough to question if it's placebo or real). But the AKG Q701 in contrast gets a significant bass boost from the 6.35mm output. Enough to change their tonal character from a tad bit bright/lean, to having a bit
more bass than perfectly neutral. So yeah you might want to experiment and see which you prefer, might not even be the theoretical optimal output!