AHA! Now THAT'S some handy info. Yah...I've never actually used tube+ on the iCAN, so I won't be using it in the iDSD either. I haven't even hooked up the iDSD yet...just got it and wanted to ask questions before I delve in....So many questions (SORRY!). If I understand correctly, do you recommend leaving the iDSD in Solid State at all times and only changing between the three modes (SS, Tube, Tube+) on the iCAN to suit my taste? I'm assuming the gain doesn't matter on the iDSD in DAC (Hifi Fixed) mode, correct? I normally listen via USB from my PC, so I'm curious which filter setting you think would be the best between Direct, PCM Upsampling (with GTO filter?) or DSD remastering...and on that last one...does that only remaster DSD or does it upsample PCM to DSD quality as well? Lastly...what way of listening is most preferred for SQ between connecting to a PC via USB or inserting a memory card loaded with music and using the app? I'm sorry...when I see knowledge displayed I MUST ASK!!
Happy to help

more thoughts below..
"If I understand correctly, do you recommend leaving the iDSD in Solid State at all times and only changing between the three modes (SS, Tube, Tube+) on the iCAN to suit my taste?" Yes, that's my thinking.
"I'm assuming the gain doesn't matter on the iDSD in DAC (Hifi Fixed) mode, correct?" Correct, the gain only matters when a headphone is plugged into the iDSD, and it only influences output gain on the headphone out.
"I normally listen via USB from my PC, so I'm curious which filter setting you think would be the best between Direct, PCM Upsampling (with GTO filter?) or DSD remastering..." There is no "best" setup, that is both the magic and curse of the Pro iDSD. It's magic because you can change the sound to your liking. It's a curse because what you like depends on what music you play, how your mood is, if your ears are tired or in super power mode after a good night's sleep, and what your brain tells you at that moment.. Maybe it's best to write down a few combinations and then switch between them if you feel like it. For example:
Filter / DSD remaster / Amp setting (if you use iDSD + iCan change the amp mode on iCan only)
1) TA / 1024 / Tube+ => sparse arrangements, classic music, voice-focused: brings out very emotional voices, a lot of treble and mid detail, and has the "large hall sound", similar to a concert hall. I find listening to Leonard Cohen's albums with this setting very enchanting. For other music with more instruments it might be too much "echo effect" and too forward.
2) BP / off / Tube => I call this the "what hifi mode" as this is their choice; works for most music, is very neutral with a hint of warmth from the tube amp.
3) BP+ / 512 / Tube => somewhere between 1 & 2, if you can't decide
"and on that last one...does that only remaster DSD or does it upsample PCM to DSD quality as well? " DSD remaster works for PCM signals.
"Lastly...what way of listening is most preferred for SQ between connecting to a PC via USB or inserting a memory card loaded with music and using the app? " I prefer USB as I am using Roon.
Hope this helps.