An intersting post that has created an excellent debate.
Here is my view on this - your choice of amplifiers is more of a lateral move than an upgrade. Your present Onkyo is a decent piece, and to upgrade to a Rotel or NAD would be noticeable, but not significant. You may want to aim a little higher if you are able to justify the expense.
As for the speakers, JMLabs make a range of speakers, from the affordable to the astonishingly expensive. As with most mfg'ers who have a vast line, some are better than others. They do make some excellent models, and some....less than excellent models. In your pro/con debate, you seem to have chosen the winner, so you should go with that.
Your main area for upgrades as I see it is the CD player. Multi disc units are convenience appliances, and single units that are better suited to your task are available. This of course depends on your needs/wants, and maybe you want the multi disc unit. This unit would fit with your amp in the upper end of lowfi, while your speakers are in the midfi realm...to use your terms and scales.
The many debate as to whether your speakers would reveal a difference using better amps, CD units, etc is interesting. The answer is a definite...maybe - depending on the level of the replacement pieces. Simply stated, by replacing ANY piece with a far superior piece, the difference should be audible. Yes, you should hear the audible difference of a better amp, and it would not be limited to a cetain price range/quality build of your speakers. Having been in this business for many decades, I can state that expensive speakers and mediocre source/amps is less satisfying that the opposite. I have 2 systems of differing quality/price points. By moving pieces in and out, I have proven this to many folks who ask the same quality.
But your mileage may vary. You did generate a very interesting, eloquent debate!
