I bought the 3100 combo already and very happy with it. I may upgrade it to 009 after I graduate from college.

I bought the 3100 combo with the 252s amp. Vocals are much more transparent and uncolored, kinda understand why some say it's addictive.

I have the L300 but I have never own any other flagships so I cant compare. But it just blow away all mid-tier traditional headphones Ive ever heard like hd650, ie800, k712, etc.

Okay thanks a lot. It is just impossible here in new york to demo stax.

stax only has three tiers right? entry tier: l300/500 mid tier:l700 and 007 top tier: 009

No.

Why are you limited to sennheiser? hd650 probably has the smoothest vocal.

T5p seems to meet all your requirement. One of the best portable closed back.

I had it new for $50 from newegg a year ago. 9500 definitely does not worth 200 bucks, $100 max.

just a small usb dac will eliminate the hiss, I had the same problem before.

Da3 has even lower floor noise, it should do the job.

I only use it with iem to get rid of hissing. Doesn't sound that much different than my laptop.

You only need one if your iem hiss with your android phone. I use the da2 to drive my im02 when I use it with my laptop only. Da2 has absolutely no background noise, but that's all it does, I dont hear any difference in sound between da2 and my galaxy s8.

Sabaj Da2?

Have you try out msr7? At its price range, it is very detailed and clear with reduced bass.

I use ie800 with my galaxy s8 and it sounds pretty good, no distortion whatsoever. I dont think an dap is necessary, your iphone x should be more than enough to drive them.

k712 is now 340 dollar in na, just 50 bucks more than hd600. To my ear, msr7 is precise and detail enough in its price range, I dont think there's worthy upgrade in the 300 dollar range. You might as well save up more money before you upgrade your msr7 unless you dont like its sound signature.

Only 2. One iem and one full size open back :)

What do you mean by natural sound? I never heard both of them but by measurements, XPT100 seems more balanced. https://www.innerfidelity.com/images/NVXXPT100AngledPads.pdf https://www.innerfidelity.com/images/AudioTechnicaATHA900X.pdf

sony's mdr-v6 fits your description pretty well, except that it is a closed back. I have both 9500 and v6 and they do sound similar.

You are going to notice a big change in audio quality, but that is not really an upgrade it is just different. If you want something similar to msr7 but better, go for k712 pro or entry level stax like l300.