There likely is a bit of a V to Wanker, so if Nari would happen to have good mids, that could be the explanation.

SSL2 is 10dBu so thats like 2.4V? For me that is too weak for HD600, it sounds boring, likely due to dynamic range.

Quedelix 5K isn’t particularly powerful either, and if I recall right it measured rather mediocre when it comes to SiNAD.

While I don’t really care too much about SiNAD and inaudible noise ranges, the power is an issue. Both can be improved for cheap though. 85eu Topping DX1 Pro which is a DAC/Amp combo measures a lot better, and has more power, although for HD600 I would like to have a further dedicated amplifier connected to it (like Topping L30 II, JDS Atom Amp+ or even just cheap Douk U3). For one device 150eu Topping DX3 Pro+ perhaps might suffice :P

The difference for me is between listening and dancing :P

As mentioned headphone amps usually are designed for 2V signal. If the voltage is too high then the amplifier input will clip (distort), unlikely to be the case for Apple dongle since its 0.5-1V. If the voltage is lower then the output from the amplifier will be lower as well as the gain is a ratio (Av for voltage).

So when you are stacking amplifiers then you are facing an issue of matching output of preceding one with input of the following one.

Another reason why using multiple amplifiers is not advisable is because every gain stage adds its own noise, and more importantly harmonic distortion. This is actually a trick used by some electric guitar players to get a highly saturated driven sound from a solid state devices :P However this is hardly an issue given how low noise and distortion is from modern headphone amps.

Also all of that is kind of relative, since DAC chips can output 2V, and dongles like Apple usually output straight from the DAC chip, similarly to how the same chips would output to Line Out in a desktop DAC. To begin with a chip is an integrated circuit, which is made of parts like transistors and contains some amplification. So the idea is more like “do not stack too many of gain stages, especially powerful ones” than “never connect headphone out to the amplifier”.

I think it is. I’ve first run into it with Apple dongle (CS46L06) which sounded like it had boosted harsh treble when I connected it to USB C in a PC. I’ve not heard that when I recently connected it to front case USB A of the same PC.

Also my CS-Pro CS43131 dongle seems to sound differently from every device, and is just confusing me a lot.

I mention that because I recall some discussions about CS43131 implementation in dongles being sensitive to USB power, and because Shio runs CS43198 which is kind of similar.