Rene, Welcome to Audioholics.
You can operate the KEF RS 103/4 speakers with your Onkyo AVR provided that you play them at a reasonable sound level.
If you follow my suggestion of not putting the volume too high, you shouldn't have any problem.
He would have to play it very softly indeed. Those speakers are known amp busters and need beefy amplification.
Here is the impedance curve.
The impedance dips below 4 ohms. It is pretty much four ohm from 60 to 600 Hz which is the major power band. Worse the phase angle is in very negative territory where the impedance is low. So the amp will see a lower impedance than measured below 100 Hz until the tuning peak.
I would not recommend driving those speakers with that receiver.
As to whether the Q90 is the better speaker, the answer is probably. In addition they would be an easy drive for his receiver as it is a true three way with 8 ohm drivers in each pass band.
The 103/3s will have a slightly deeper bass but bloated.
These speakers came from a time when coupled cavity bass systems were in vogue.
The problem is that you can't build a decent passive speaker that way. At least the low pass, to mid must be an active crossover.
With my experiments back then I found that out quickly.
If you want low Q tight bass then you have a narrow bandwidth. If you want a wider bandwidth then you get high Q and bloated bass. Even at the crossover point of the speaker in question the crossover at 180 Hz is still too low for a decent passive crossover.
So I found that to get a response from the high twenties the highest you could really drive the bass unit was 90 Hz, and then you ended up with a Q on the high end of acceptable at 0.7.
So my speakers ended up active for the bass to mid at 90 Hz, with a Q of 0.7.
The object was to design a relatively compact monitor with a wide bandwidth to monitor live recordings.
The design worked and I still use them today. Build date was 1989. The bass is much better quality than the KEFs.
The KEFs have another issue, which I suspect is shared with both the speakers referenced.
If you look at the frequency plot you will see a deep notch out in the 9 KHz region.
I have form with this issue also. My current studio center uses the SEAS prestige coaxial drivers. SEAS make these drivers under license from KEF and there have a lot of similarities with the early KEF Uni Q drivers. They have that very same suck out in the 9 KHz region. This is subtly audible and imparts a dullness to the HF. I am certain that this problem is due to cancellation from cone reflections from the tweeter. I found this a very difficult problem to deal with.
However dealt with it was.
So in summary if you are going to use that Onkyo receiver, then use the Q90s. They are likely the better speaker anyway all round.
Both of them are pretty good speakers, although the bass bloat of the 103/3 will likely present problems in a lot of rooms.
I can find no third party measurements of the Q90s, but I would expect them to have the same HF suck out. In any event a dip in frequency response is far less noticeable than a peak.
