1dB can definitely make a difference in perception of dynamics and frequency response. Level matching is the hardest part of any A/B comparison.
Soundstage is a function of microphone placement, the mix and your room acoustics. An amp can't affect that unless it's defective and has tons of crosstalk between channels.
As for the dynamics, did you have both amps set for 8 ohms on the speakers? If the Onkyo defaults to 4 ohms, that would cause a significant difference in dynamics.
Frequency response imbalances are the most likely differences between amps, especially with an AV amp that is designed to be equalized to the room. But equalizing both amps to the same curve (or EQing one to match the other) should produce identical results. That's why earlier I asked you to try adjusting the tone controls on the Onkyo to see if you could match the sound of the mono blocks. (I'm assuming you don't know how to use the equalizer built into the amp).
The Onkyo has a ton of tone adjustments that the mono blocks don't. You need to work in the menus a bit and figure out how to set all those settings so it produces the same output as the "one size fits all" setting of the mono blocks.
P.S. Was your test double blind?
In principle, you are right.
In practice, amps DO influence sonics. I thought that channel separation of 90 dB 20Hz-20kHz was enough, that any further improvemnt will bring nothing.
Boy, was I wrong. I went from approx 90 dB to approx 100 dB (or more, could no longer see the crosstalk on the oscilloscope ). It WILL NOT show up on A/B test - because the switch/difference is too small in duration. After listening to a "90 dB separation" amp and switching to a "100 dB separation" amp , long term impression is one of simply more close to the sound heard live - tiny details we tend to glaze over in quick A/B ing emerge, and they are sorely missed when going back to "only 90 dB separation" amp.
In truly critical positions, I insist on dual monaural approach. Although NOT to the degree/extent of one stereo preamp from Mark Levinson - SIX boxes, phono, line, power supply, duplicated for stereo - the whole preamp had exactly 4 controls, volume control and input selector/each channel - NO on/off switch, it was on if plugged into the wall outlet.
I ALWAYS tend to listen to the long term effects of so small diferences as described above while doing the chores, like dusting. etc. If it does feel overall more natural, comfortable, etc while listening in a totally relaxed mode, it IS better.
I am certainly not against A/Bing, precise level matching, etc - but it is not the ultimate indicator whether the device under test is truly superior or not. No prospective buyer in the market for new violin ( or any other instrument ) is likely to decide between two comparable instruments in A/B lasting only few minutes...