Hmmmmm, I don't know if I agree with that statement, Wolf, but, in all honesty, I don't know if I actually disagree. I think affection means different things to different people and, personally, I've never had a single tiel that I considered 'affectionate'. I had a few hand-fed ones that were lone pets and very used to been handled and they were sweet little things (would step up, ride my shoulder, even eat apple from my fingers) but none could ever be compared to, say, Freddie (LSC), Tobita (female Senegal RIP) or BabyBoyd (female Lesser Jardine). These birds would proactively look for caresses, kisses, hugs, etc while none of the hand-raised tiels ever did but, then, my tiels have always (and always will) live with other tiels so I guess it is possible that one can have a single very affectionate tiel while, if you keep them in a small flock (as Glenn does -btw, welcome to the forum, Glenn!), they will be tame and friendly but not really affectionate.
In my personal opinion (and in Nature's opinion, too

) tiels do much, much, much better when in a small flock so I think you have almost the ideal situation as far as the birds are concerned (the 'almost' comes from the fact that you clip, something I do NOT agree with no matter what the situation) and I would just let them be tiels and don't worry too much about 'working' with them.
Now, yes, cockatoos are, most definitely, VERY affectionate birds. But in order for the situation to work for them, you need to be a stay-at-home person. I don't think there are two ways about this when it comes to toos because all the ones I've known that are single birds and have humans who work outside the house scream (and most barber or pluck once they got to a certain age). And, when I say scream, I mean LOUD pterodactyl-like calls that feel as if your eardrum is going to split and which can go on for looooooooong. I took in a screamer some time back, Freddie is a 21 male Lesser Sulfur Cockatoo and he is a sweetheart (kisses, hugs, tickles, the works!) but he was also a screamer -his folks worked outside the house and he was by himself all day long, interacting with them in the evening and night so he was overly hormonal, lonely and frustrated -thus the screams. It took me 10 months to get him to do his calls only at twilight (and let me tell you that the first 5 months were murder!) but, every now and then, when I am in the birdroom and he gets excited, he still screams - no biggie because all I have to do is put my hand in front of his face to distract him and he stops. But, two or three days ago, he was perching on my shoulder and screamed -something that had never happened before- and I was blown away by the sheer potency of the sound! I could actually feel the sound as a vibration in my head! Like thunder when the lighting falls very near you! That's how powerful and loud their screams are. So, if you are thinking about getting a too, please consider that, if you work full time outside the house, you will have to make arrangements for him/her to have company during the day.