After more reading round I've found
this post which seems to do some "RMS to listening volume" calculations for speakers rated at less power than the amp.
Someone please correct me if my calculations are way off here or make no sense, but taking those and adapting them to my potential set up, I get:
Denon AVR X-2300W amp: 150W x 8 ohms = 1200
Current Onkyo speakers: 120W x 8 ohms = 960
(960 / 1200) x 100 = 80%
So if this is correct that I could give the speakers 80% of the amp's power before damaging them, then that's great as I've never had my current amp louder than 50 on its 1-99 volume scale - therefore I should be able to keep the Denon at sound levels matching the current ones I listen at, while leaving loads of headroom in the speakers before potentially damaging them, correct?
Does any of this sound right or am I making no sense? haha
Edit: Argh, the speakers are rated 6 ohms impedence. Therefore I assume that reduces the above calculations - to what presumably depends on the amp - for which the specs say:
Power Output (8 ohm, 20 Hz - 20 kHz, 0.08% 2ch Drive) 95 W
Power Output (8 ohm, 20 Hz - 20 kHz, 0.05% 2ch Drive) -
Power Output (6 ohm, 1 kHz, 0.7% 2ch Drive) 125 W
Power Output (6 ohm, 1 kHz, 1% 1ch Drive) 150 W
I'm assuming I go with 6 ohm as that's what's rated at its claimed 150W, so:
X-2300W Amp - 150 x 6 = 900
Speakers - 120 x 6 = 720
(720 / 900) x 100 = 80%
So no change in the volumes I have to stick to, which should mean I have no issues with volume potentially being too loud for the speakers - I hope!