Thanks for taking the time out to test different sources, it is greatly appreciated and helps immensely
As with all things audio, even though IMR IEMs are highly configurable, personal preferences, music choices and synergy are key to how much value products have to each individual at a subjective level.
From what you describe it seems that the Opus Mia has a good degree of transparency (albeit with the IMR's house sound) and seems to enhance the characteristics of its source, which is exactly what you want at this level.
Whether an individual has the capacity to hear the quality of having, for example, electrostatic and planar drivers in one IEM perhaps depends on the quality or ability of that persons listening. Many people hear but very few people really listen
Absolutely but then listeners are interested in the presentation within their auditory range and this varies immensely not just on individual's anatomy and preference but also choice of music and recording quality eg. VHS looks terrible on HD TVs because you begin to see artifacts, forgiving old SD TVs look better. The same is true with audio given the varying mastering of tracks. The key is choosing the right tool for the job.
I understand exactly what you mean about different volume levels bringing out different frequencies and that's not unexpected. The high subbass response being mentioned in these early Opus Mia impressions is a concern for my preferences but I'll wait to see what the tuning nozzles can manage on the Semper.
What I have specifically experienced with these types of hybrids, where no crossover is used, is that certain drivers have an upper limit to how much power they can handle and thus more power ends up pushing the other drivers in the configuration while a certain frequency range plateaus, not to mention sensitivity will be different for mixed technologies. In my experience it seems to relate to the passive electrostatic drivers and likely due to a limit on how much power their internal driver circuits can handle. They also seem to have a threshold before really driving properly and I am a very low volume listener.
Crossoverless multi driver IEMs aren't unusual but they are uncommon because crossovers give a much easier level of control when tuning as well as control over time domain (which is usually managed by implementing different length voice tubes per driver to ensure that frequencies reach the ear at the same time without phase issues). The few manufacturers who excel at crossoverless multiple drivers is predominantly limited to those who are able to design their own drivers to spec and that's less common (and more expensive) than buying off the shelf. Clearly Bob has significant experience here.
Bob has mentioned to me a few times that he does not like to listen to offerings from other manufacturers (he didn't want to try the headphones I had on me the one time we met in person). I can understand that however the Opus Mia and certainly The Semper are pitching at a very high level, where expected compromises are far less.
Early impressions for the Opus Mia are extremely encouraging. I'm not expecting the Semper to bring much more to the table than the Opus Mia but at £1100 there is promise to rattle the cages of the +£2000 IEMs in users' collections with the potential to be able to tune the Opus Mia/Semper to mimic the strengths of each and save them (or me) carrying around 4 IEMs when I travel (coloured for electronica, accurate for instrumental and voice, wide and resolving for large orchestral, intimate for acoustic etc)... Sort of a Super Head & Shoulders of the IEM world (take two bottles into the shower?!). I really don't like carrying lots of expensive equipment but my love of music far exceeds my fear of personal safety when I travel. The sad thing is that if I am robbed at gunpoint, I won't be able to replace the Semper.
On a side note; with regards to the titanium driver in the questionnaire IEM (yes, what you have quoted for the configuration sounds correct), I can only speculate that it would be a preference to those who are not fans or the treble response of the piezo ceramic driver used in some of IMR's other offerings.
Cheers for your time and information and I'll await the Semper to see what it brings to the table. Potential for something very unique.