Thanks for the responses to my ramblings. I did a bit more testing and messing around to get to the bottom of all this and have come to some conclusions and workarounds and will probably carry on and just not cut the cord as much as planned. In my testing, I couldn't help but feel like the quality on wired dwarfed my wireless sets significantly. I don't know what's going on with the bluetooth volume, but I do feel like they have the desired bit of extra oomph playing the same files through plex to my samsung phone, but the "problem" files are still an issue there too. I tried swapping codec's and found volume ceiling wasn't affected. The capabilities vary on the headsets, so I can't try them all unfortunately.
I have the following headsets and amp to use for testing.
-Senn HD650
-1More Triple Driver IEM
-JDS Labs C421 Headphone amp used for HD 650's
-Senn PXC 550 Wireless BT 4.2
-1More Stylish Wirelss BT 5.0 Earbuds
-My wife's Tribit Wireless BT
Bluetooth volume is definitely lower across the board compared to wired on the M9, and significantly so. All 3 headsets need to be near maxed out at 110-120. Tribit is loudest but also lowest quality of the 3, 1More Stylish Earbuds are next and PXC 550's the lowest. I've since read a lot of complaints of the PXC 550's being too low in volume, so that seems to be compounding the issue here. I still feel that on all 3, there is a major lack of headroom that seems unacceptable to me.
I let my PXC's battery die, which allows you to wire in passively. Amazingly and annoyingly, the only way to do this is to kill the battery, because the "off" switch is turning one of the earphones. They got much much louder and sound better wired in passively, but before the battery died were no louder than wireless via BT. If I can't live with the volume ceiling on this set in particular, I will sell them or abandon bluetooth capability, let the battery die and use them as a wired set only.
My 1More wired IEM's are loud. I assume they are very easy to drive. I have to have the volume in the 60's or 70's on high gain, with max on trouble files would be low 80's. A bit higher on low gain, but not much for these. Would blow my ears out if I had it near the levels I need on bluetooth headsets.
My Senn HD650's definitely need more to drive, but that's expected. I'm hovering around 90-105 without the amp in the chain. With the amp, I can use that to drive them beyond what I need. I was also able to easily drive them via 2.5mm, but I typically leave my 3.5mm cable in my 650's because that's the only connection on my amp.
The DB level on files is huge here. On plenty of stuff in the 95-101 db range, I can be at 110-120 on the M9 and be happy. The mid to lower stuff is the problem. Files in the 91-94 db range are okay and only seem to lack a little bit of oomph on the bluetooth headsets. The stuff in the 83-90 range though, it's intolerable to me and I just wouldn't listen to it on bluetooth set because of it.
I have solved this a little bit between the two apps I use on the M9 and having replaygain tags saved to all my MP3's. I do like bouncing between two apps to keep things fresh and they each have their small quirks.
I have the Onkyo premium apk app that has Replaygain functionality and preamp settings, so I can crank the preamp as a compromise. By only adding RG tags and not actually permanently altering the files db levels (which I wanted no part of, obviously), it then gives me the option of turning off the preamp in Onkyo or launching the MediaMonkey app that doesn't read Replaygain tags at all. Then, when I want to listen to the files on my wired sets, I can still choose to listen to them in their "purer" form without RG being added.
With BT being a "lowered" experience from wired sets ime anyways, at least between the ones I own, I'm not too concerned about some mostly inaudible clipping on files from RG preamp being raised. These headsets are for out and about and convenience mostly.
I have a lot of FLAC files as well, but anything I have in FLAC, I have in 320 Mp3 as well, so with bluetooth experience being what it is, I'm again not too concerned and will leave the FLAC listening for the wired sets.
In the end though, I wish I had a better understanding of the issue with bluetooth volume here. In the last few days, I have read a ton of people complaining about low volume on their bluetooth headsets with no answers beyond the commonly circulated ones. that didn't help me. It doesn't seem to be my ears as my wife concurs. I could just have 3 generally low volume sets in the end and will surely test another when I have a chance. If we can control volume on the M9 separately from my headset though, why couldn't they just add a couple levels on the volume meter? I realize that's a very primitive way of putting it.