The speakers are certainly in a far superior league than the Onkyo. I think that given their sensitivity you might be able to get away with it since your amp is a high current design. It depends a lot on how far below 4ohm the speakers actually go and how hard you intend to drive them.

I would really, really, really have to love my Onkyo to let it stop me from buying such fine speakers.

I have those Sonus Faber speakers. While they are nominally 6 ohms, they do drop to nearly 3 ohms between 7K and 10K. They are short on extended bass, but in every other sense wonderful. Save your money for better amplification.

I drive mine with a Musical Fidelity X150. It's a dual mono integrated amplifier - high current (50 amps peak to peak) - modestly priced and very well thought of by reviewers. There is also an X80 available which might be adequate depending on your taste in music. The 150, I think, handles orchestral peaks more commfortably.

Some say Sonus speakers sound best with tube (or valve if you prefer) amps. You might want to audition tube vs. solid state before buying.

I can assure you, however, that if you go with the same setup I have used for the last couple of years, you'll be very satisfied. You'll hear things that weren't there with your current receiver.