@mozartfan It would be really cool if you could attend an audio show or maybe there is a great dealer near you. You really owe it to yourself to hear the differences between all of the topographies so that you can find out where you really are.
I remember vividly many, many moons ago when a very helpful dealer played a series of amplifiers for me, several tube and several solid state, in descending order of price. He played them through a very popular speaker at the time. My conclusion then: I couldn't hear much (if any) difference. He agreed. That made me feel great.
Then, he repeated the same exercise with a pair of exceptional speakers...everything else the same. Same descending order of price, etc, etc. The sonic differences between the amps was striking, astonishing actually. The dealer knew what he was doing. He was a WWII era radio operator, had started a radio repair shop post WWII and evolved it into a fine hifi shop which made it into a third generation. His moral to the story was to pick the speaker you like and then if you can't tell the differences between the amplifiers, relax and buy the least expensive amplifier whose sound met your goals. His BIG advice though was just because you can't hear the difference doesn't mean there isn't a difference, it just means your speakers aren't up to the task. He added that if I ever intended to upgrade the speakers then always buy the best electronics that you can afford at the time to ensure the speaker upgrade down the road would have the chance to pay dividends.
No offense intended @mozartfan but if you can't hear the differences of all of the vast array of amplification varietals that you have implied you have experienced first hand, then it isn't the amplifiers, its more than likely your speakers aren't up to the task of revealing the differences. You should be celebrating though! If you like your speakers and all of the amps sound similar to your ears and you are happy with the sound then pat yourself on the back!!! You found happiness and can achieve it for an exceptional value. @atmasphere is an amazing resource, he is an absolute expert when he is talking about amplifier design (not just OTL's but generally all topologies). He would be the first to tell you if you are happy with the sound, then stop. He would also tell you that amplifiers sound different and if you don't hear the differences then its probably the transducers.
I recognize, however, that you may be more likely than not going to argue with me and say that it can't possibly be your speakers...in that case, enjoy the solo wrestling match. Warm regards and happy holidays.