^^ That's not the whole story.
Like all speakers, the 683 S2s have an impedance that varies with frequency. At certain points the impedance measures greater than 20 Ohms. At other points it's less than 4 Ohms. It even dips below 3 Ohms at a couple of points. But it's not just the Impedance but the Phase Angle as well.
"Okay, what the heck is Phase Angle?"
Because impedance isn't just about resistance in Ohms, but also about induction and capacitance, then what the amplifier sees as the load from the speaker is far more complex than a simple 8 Ohm fixed resistive load. This means that when the amplifier is applying the varying voltage across the speaker terminal that represents the music signal and its volume, and the speaker is reacting to that by drawing current from the amp, then the voltage and current curves will separate. The result is that as far as the amp is concerned, the more the two separate then the greater the nett cancelling effect. At 45 degrees of phase angle, the amp has to deliver twice the amount of current to maintain the same volume output by the speaker compared to 0 degrees. A practical display of this effect is in how hot an amp runs with a speaker that's a "difficult load" because of the phase relationship.
If it helps, you could visualise phase angle as what happens when you have to push something. Say your friends car conks out and you need to push it to the garage. The sensible thing to do is to push in the direction of travel. That way, all your energy is going to move the car where it's heading. This would be the equivalent of a 0 degree phase angle. Now let's say for some bizarre reason you decided to push the car from the rear quarter, so you're pushing at 45 degrees to the direction of travel. That would mean that half your effort is going to move the car forward, but the other half is wasted trying to move the car sideways. The result is that at a Phase Angle of 45 degrees you have to work twice as hard to move the car the same amount of distance as at 0 degrees.
All speakers exhibit this odd phase angle electrical phenomenon. Some speakers do it a bit less, and others a lot more. Where a speaker spends a lot of time going to either a positive or negative phase angle of 45 degrees then it's said to be a difficult load. Where the adverse phase angle coincides with low impedance, then that's where the amp is going to have the hardest time keeping up with the power demands.
There's a review of the 683 S2 speakers from Stereophile magazine. I've linked to the last page which includes the lab measurements the mag did. https://www.stereophile.com/content/bowers-wilkins-683-s2-loudspeaker-measurements
The first graph shows you the frequency in Hertz across the bottom scale, on the left scale is impedance in Ohms and on the right it's phase angle. The curves aren't bad. However, the speaker does spend a chunk of time below 4 Ohms between 90Hz and 700Hz. There's a lot of fundamental energy in music and film soundtracks in that region. You've got most instruments in that range and pretty much all human voice except perhaps for Barry Whyte's speaking voice. That's one only whales can hear
The other thing of note is that the phase angle and impedance curves cross at approx 85Hz. Looking at the graph it appears that the impedance is 4 Ohms and the phase angle is roughly 55 degrees. Again there's a lot of soundtrack/music with content at that frequency, so the amp is going to be working quite hard at that point to move those bass drivers
There's more reading here on phase angle and what it all means. https://www.audioholics.com/loudspeaker-design/understanding-impedance-electrical-phase/page-2
"So what's all this mean for sound quality?"
The tell-tail signs that an amp is running out of steam are a hardening in the treble, a shrinking soundstage, or things just sounding a bit congested and not so easy to follow when the music gets busy. To be fair, the Sony has received some comments in reviews that the soundstage isn't all that wide or deep to start with.
Honestly I am not after volume, I’m after quality sound. I doubt I’ll ever get anywhere near the max the amp can deliver to the speakers.
I do appreciate that the more expensive the amp is the more likely I am to get quality sound. But the Sony 1040 was well reviewed at the time and I’m sure they weren’t testing it on £10 speakers
You will get quality with the B&W speakers; I have no doubt about that at all. Also, I agree and I think I've written previously, the Sony was well reviewed generally. Within its peer group it did rather well. That doesn't mean to say though that it can't be bettered.
AV receiver performance tends to run in cycles. When new features are hot off the press then I get the distinct impression that a lot of the R&D budget gets switched from sound quality aspects and on to scaling or networking or 4K or whatever the feature du jour happens to be. Once things settle down then the focus goes back on to sound quality for a while.
Right now it's ATMOS/DTS-X and Dolby Vision/HDR. The video side of things isn't so much of a concern other than the chips and licensing costs nibble away at meagre budgets. The bigger worry is all the additional amplification channels required to make ATMOS etc a reality. We come back to this idea of a transformers power as a limited size cake. Stepping up a model or two in performance is inevitably going to include the almost mandatory additional 2 or four ATMOS channels feeding from the single cake. That's a bridge to cross when you come to it. Keep an open mind to buying an amp/receiver with pre-out sockets. That's the way to add extra amplification simply.
In the mean time, you have your 7.1 ch Sony. I think you said you've been running it in 5.1 mode with the Bostons. If your priorities include music, then I think 'd be tempted to stay 5.1 rather than go 7.1 until you can afford a bigger cake. You might not be running the Sony at a high volume setting, but even so, there are dynamics in music and sound tracks that need instant power on tap, and the speakers themselves will be interacting with the amp and drawing more current at certain frequencies than you might expect for the volume. If you hear the sound stage getting smaller, or things just seem more difficult to follow, then that's a sign that the amp is feeling the strain a bit.
Having said all the above, I have run my JM Labs Electra 905s with £700-£1000 AV receivers and AV amps for the past ten years or more. The 905s are a bit more efficient than the 683 S2s (91dB vs 89dB) but they don't go down quite as deep. That means they're not pulling quite as much juice for the lower registers. I have run them from a lovely little 35W/ch Creek stereo amp and it did okay. Swapping out for something altogether more powerful with a decent current reserve showed the speakers in a different light. My AV receivers have tended to be a bit more juicy than the Sony. The last two both had power consumption figures north of 550W.
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