The substrate that the layman perceives and that physicists study.
If the substrate is phenomenal consciousness then there is subjective experience and therefore a subject - so if we have a subject with phenomenal consciousness having subjective experience, then we have a mind. The substrate is mind.
However that conflicts with what you have said about minds arising from (a la "whirlpools") the substrate - but it would fit with some forms of Idealism in which individual minds (little minds) form from Mind (big mind) and then are absorbed back into it. It's not clear what experience(s) is available and not available to Big and little mind (you've said this is a challenge to your view - why all of mind is not accessible - but just there you should look to the evidence for non-local mind ... )
There is also a conflict as you have referred to "subjectless experience" but denied experience without a subject.
Whatever the case, phenomenal consciousness and subjective experience can't be pried apart because they are different ways of speaking about the same things - phenomenal consciousness is a container for subjective experience(s), subjective experiences are what phenomenal consciousness allows a subject to have.
Phenomenal consciousness doesn't "hang in the air" absent "subjective experiences" any more than "running" hangs in the air absent "physical abilities" - there is no running without an instantiation of running - i.e. a runner and physical abilities are what allows one to run.
http://www.univie.ac.at/constructivism/journal/articles/10/2/267.solomonova.pdf
“Since consciousness by nature is experiential, and
experience is primary and ineliminable,
consciousness cannot be reductively explained in terms of what is fundamentally or essentially non-experiential.”
that said,
all that said:
"... Thompson directs the reader’s attention to an alternative: in Indian tradition, consciousness is separated into gross and subtle consciousness. The gross or coarse consciousness is understood in the sense of general attention and awareness of the self and the environment,
whereas the subtle consciousness is a substrate energy, a source of gross consciousness. According to yogic and Buddhist meditative traditions, one can become aware of subtle consciousness in some dream experiences, at the moment of death, and following rigorous meditative training. This perspective opens avenues for new kinds of questions for the science of the mind. Can one train oneself to appreciate the qualities of subtle consciousness? Adepts of dream yoga would say so. What kind of electrophysiological activity would reflect the state of awareness of subtle consciousness? And finally, what kind of world opens up in the state of awareness of the subtle consciousness?
In other words, is awareness without object possible? These and other questions are common to contemplative traditions, and are now making their way into contemporary neuroscience."
But, as I recall, you (at one point) objected to "awareness without object".
And I posted an article (by Thompson, no that other guy) about continuity of consciousness
a la dreamless sleep as a state of consciousness.
So there are avenues to explore.