Thanks for your amazing detailed explanation, I'm very grateful and impressed.For #1 - Back in 2021, I noticed that the supply of JAN5899 tubes had "dried up", and the portable amp manufacturers had switched to JAN5784 tubes.
I posted about it on Page 45 of this thread - https://www.head-fi.org/threads/portable-tube-amp-from-china.626237/post-16095516
For #2 - The TAP-1S is fully balanced, and has signal phase splitters, which allow either single-ended or differential inputs to produce either single-ended or differential outputs. This is unlike some other portable amps which will only produce the output that's the same as the input. The TAP-1S also has the External "Desktop" Power Supply, which allows it to produce almost 2.5W/ch output. The TAP-2 produces 0.5W/ch output.
For #3 - I don't have any OPA2132 op-amps, but I do have MUSES02 op-amps, and suspect that the MUSES02 would have a "warmer" sound.
For #4 - If I didn't need balanced output, and wanted a small tube hybrid Amp - for convenience of use, I would select the TAP-2 - since it uses a standard USB-C for charging, and the B4-X uses a special 12.6V Lithium-Ion charger. The B4-X can not be used while it's charging - when the 12.6V input is connected, a relay disconnects the DC-DC supplies that power the tubes & op-amps, so the battery can be charged.
So between b4 x and tap2:
1. B4 X I manage to speak to the shop and they can modify it where it can be used while charging at the same time. The only concern is noise. Does tap2 have noise when in use? (during charging and not charging)
2. I'm able to find USB to12.6V charging cable. That solves the portable issue too.
So base on these info, any other things that may separate the two? Such as more power, better tube sound, better layering/soundstage?
I can see both device can be opened and change Op amp.