Always a personal perspective, so bear that in mind.
The 40a’s were lovely looking (the fishnets, we called them, cos of the coarse grill) and sounded very nice - good pace, clear mid range, decent treble with no brightness, and ok bass. The 50a’s were uglier but sounded better - midrange even sweeter, with detail, clarity and musicality all clearly there. Treble was again good, bass better - deeper yet more controlled. Nearly did a deal on those, and the decided that in such a price bracket I owed it to myself to trial some others. Cue one trip to a dealer in Kent and two trips to Chester for some listening. Heard Magicos, Kudos (various), other dynaudios, b&w’s, etc.
Unfortunately for me it turns out my wife is much better at assessing speakers than I am, and for some she’d say no way before a few bars had passed - and was right though it took me a few tracks longer, usually.
We didn’t get on with b&w’s as they are too bright for us - I can see that if you like that detail in the high end they may suit you, and looking like a Darlek is a positive, but not for us. Magico’s (various) not for us either - found them too bass and loose. Again I know others have chosen them and might disagree - and I don’t dispute that they may well be best for them just not for us. Confidence 50’s were the last to be tried, and were excellent in the shop. Clearly a step up. Went back to the Kudos then back to the Confidence’s - they have controlled responses, are fun to listen to, superb treble with all the detail you could want without making your ears bleed. Musicians leap out of them - one of the interesting things is that from the room next door, they get closest to you thinking there’s a band next door.
We did try the Confidence 40s which are a bit smaller but the 50s seem the sweet spot in the range - they are more, but you get more, too, and if you have the space they are great (room is 6x5m, vaulted ceiling). They have run in now in our room, and are sounding better and better (though some of the other kits hasn’t yet (552…). Need to get the curtains back up for optimal sound, but it’s lovely. Sitting here with ‘Letter to You’ on quite loud.
There are some similarities to the sound of the 50a’s - its musical, it’s clear, it’s not fatiguing to listen to, and works at a range of volumes. It offers just a a bit more in everything, really. I like the looks, and whilst tall they are slim and fit with the room. Modern compared to the ATC design. ATC matches it in mid-range, or gets close, but loses out top and bottom.
If we’d decided to stick to the budget, then the Bartok into 40a’s would be a great system, and could be later upgraded with a preamp. We decided to blow it as we could clearly hear the upgrades.
One other observation - mostly demonstrated to me by my wife (who is now into hi-fi, thankfully) - it’s important to go and listen to the music, not to the hi-fi. You can tell yourself all sorts of stories and try to be overly analytical, and whilst there’a place for that, a good system will sing to you right away and you’ll have a pretty good idea.