You can always rationalize a reason why the new one is different enough to make it nessidsary… I have closed back and open back headphones, but those are both dynamic drivers, so of course I need a planar… So you get some Sundaras, but maybe now I need a closed planar…

Particularly with digital sources, unless you are going out of your way to use a crappy source, differences are marginal. Accurately Converting a digital signal is a very easy and cheap thing to do with modern technology, isn’t much room for improvement by spending more. The THD of the DAC circuit in a $10 USB to 3.5mm adaptor is 0.01%. Which is about 1/10 the amount of distortion that would be audible even with the best ears listening to be best system.

The Signal to noise ratio of the Apple dongle is 99 dB, which means you would need to crank the volume to 99 dB to get 1 dB of unintended noise. For reference the ambient noise in a fairly quiet room is 20-30 dB and 85 DB is when you start damaging your ears, so you would have to turn the volume up to injurious levels to create 1/20 the amount of noise in a quit room. If you want spend thousands of dollars to make the noise and distortion even more inaudible than what you already couldn’t hear, of course someone will be happy to sell it to you.

With speakers, because you have mechanical motion, there will always be things flexing and rubbing and vibrating, so there is far more room for improvement. Analog, by definition requires mechanical movement, so again there is more difference between the signals produced by different analog devices.

Objectively, The AXA25 has 5 inputs vs one for the Fosi, so if you are using more than one source, you will need a pre-amp . whether or not more power adds anything to the listening experience depends on the room and speakers being used. I would contend that the area of overlap is pretty small in the venn diagram of rooms where the Fosi amp will be used and the rooms where that much power is useful. Subjectively some people will prefer the sound of one amp, most probably couldn’t differentiate in a blind A/B test.

What steps have you taken to verify that conclusion?

I have a no longer supported LG G7 with a SD car full of my music collection. It has enough Juice for all my headphones except for my Modified Fostex T20. To be honest, I haven’t used it much since I got my Qudelix 5k which I use with my functioning smart phone.

Being able to re-foam and recap old speakers. I really prefer the sound of large-boxed. The sound I li

Well, you can use frequency response to tell you some things, like if they are bass/treble heavy or v-shaped response. After that when you have heard some things that reviewers have also done reviews for to gauge how much your own taste aligns with various reviewers.