George, you are correct in many of your assumptions and I'm in a somewhat similar place in my hifi choices but, under different circumstances.
I have the AKSA amp and am very happy with it. It will reveal details that the Onkyo can't. It will also do a lot of other things right as well. With the Onkyo, you will hear congestion and harshness as the volume rises compared to the AKSA. These sonic characteristics are a much more important factor to me rather than the specs of which amp can handle which load... Unless Onkyo has changed over the years, they tend to have harsh preamp sections. This weakness will be readily revealed by the speakers and amp you are considering.
Now, what's the good news? Some good pre's are cheap-ish. I just sold my preamp so, that's not an option but, if you can find a used Audible Illusions 2d, you will have a great pre. Also, a Melos MA111 can be found for around $200 that will be suitable. Those are both tube amps which will work well with the AKSA. One last option and people may laugh... is a Hegeman Hapi II pre. It's solid state and does nothing wrong in my book. It'll blow the Onkyo out of the water in sonics.
I like your speaker choice. That would be the one thing that should remain constant. Upgrading electronics is much easier than speakers in my opinion. You have a constant in your system if you do this.
If you can't afford the AKSA/cheap pre at the moment, maybe in the meantime you can get a different amp that sounds similar to the AKSA but, isn't as good. The older B&K ST-140 and ST-202 are excellent options for now. They cost from $200-$500 and are ready to play. That way you can get a good pre... maybe something like the Purist from IRD (which I'm considering at the moment) and then afford the AKSA later. This would get the Onkyo out of the equation quicker.... which to me is going to be your least favorite part of the system wherever it is.
I have not heard the Purist, though... so, it's just a conceptual suggestion. I whole-heartedly recommend the AKSA 100 w/Nirvana. It's the best amp I've heard and provides a LOT of flexibility.
Best of luck.