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Another cause for different prices is the duration of the warranty / guarantee. It's at least 2 years in the EU, but only 1 year in the US and Japan, if I'm not mistaken.

Maybe. It's not taxes or duties anyway, since those are 0% for headphones. In Denmark, the X9000 is offered for 49999 DKK, or about 6724 EUR. Still 600€ more (at 25% VAT), but maybe there are other countries with more reasonable distributors.

Stax Headphones behave essentially like a capacitor. DC resistance is probably multiple Mega Ohm. Assuming you stay within the current (and voltage) driving capabilities of the amp, things should get complex, but I wouldn't expect a change even remotely as large as you postulate. That's accurate at least within the measuring accuracy you're going to get with an off the shelf multimeter. As above, if the combined capacity (etc.) stays well within the capabilities of the amps, I wouldn't expect much of a change. There are probes with a U shaped hook at the end. If you find one that allows you to bend that hook into a shape that wraps around the pins of the plug, that would probably be ideal. You'd still have to make sure that the probe doesn't cause any shorts, so just measure on the second socket if possible.

He wants to (somewhat) objectively determine which of the amps is actually the best. To accomplish this, he absolutely has to level match them pretty precisely, and using a multimeter to do so is actually a very good choice. Otherwise the amp that is even slightly louder in the comparison will appear to sound better, masking any minor differences that may actually exist.

For the Stax amps with two outputs, all the corresponding pins are connected in parallel. Assuming your selection only contains such amps, just plug your multimeter's probes into the correct receptacles and you're done. If you still need to go with your idea with the extension cable, I think you'll only need to expose the bare wire enough so you can get the probes to contact it, so drilling a small hole with 1mm diameter would be sufficient. If you don't want to ruin a 100$ cable, just buy a pair of clamp test probes for your multimeter and attach them to the pins of the headphone plug and insert it as far as possible - it should still make proper contact. You'll need to be careful, and ensure that the probes you buy can actually wrap around the pins.

With a little patience, that's enough for a 2nd hand SRM-727. That really makes that Sirrah a bad deal.

I'm wondering if that plastic part fits into "old' 007 mk2, and if STAX is willing to sell them separately. billqs, could you take a few additional photos of that part and the port?

If the dust screen is disintegrating, it probably high time to replace those. I found the instructions for pad I had read so far somewhat unintuitive, but luckily there's a video by STAX with a thorough demonstration how to install the EP-007: https://www.youtube.com/watch?v=TyRZFD8yv8Q

Have you tried adjusting to a linear response or any of the Harman target response curves or a variation thereof?

You could still 3D-print molds and then build plugs from other elastic materials, I was thinking acrylic, sugru or silicone. Not sure if any of those are actually better than Blu-Tac. I'd buy a pair if you figure out a good material and were able to drop them off in Yaletown...