I can't comment on the KT90 tubes, but I do have some experience with 845 tubes in different set-ups, and I think it could be of help.
A while ago I borrowed a well-designed and well-built pair of 845 SET monoblocks from a befriended dealer. It had 300B's with anode choke as driver, and a 6SN7 first stage. On his own speakers they performed well. On my ESL57's they gave a very flat soundstage, same as you describe. Even the general direction of the instruments was very vague. The instrumental timbres were not natural; everything sounded alike.
Earlier I had borrowed a 300B SET (again with 6SN7 as first stage), and that gave the same results.
But: I have also listened to 845 push-pull monoblocks (Mingda MC300845-AB) on my speakers for a few hours. Apart from being a push-pull amp, the tube line-up was very much the same as the 845 SET's: Anode choked 300B as drivers, first stage with 6SN7, and no global feedback. The result was fantastic, with good depth and placement, and a very good rendering of the instrumental timbre. The most lively and involving sound quality I have experienced so far, while the other amps with exactly the same tubes were the biggest disappointments I ever had.
The only reason I didn' buy them is that I can't stand the sight, especially with their protection cages. Every now and then I still think of buying them and have something different made.
Same as other people have commented: It's not just the type of tube that means success or failure. I do have some idea what it is that makes the difference with my speakers. ESL57's are a reactive load, so the amps needs to be able to deliver a good peak current. That makes SET's not very good candidates, because they are low-powered amplifiers with a relatively high output impedance. The push-pull 845 amp had 70 Watts, so could still deliver enough peak current despite its high output impedance. My own Fisher SA16 (EL84 push-pull) has low power but with a very low output impedance, which also makes it work well. Until it runs out of steam in really big orchestra pieces, that is...
I can't really explain how this translates into things like soundstage, so it's not a proven fact but an idea that seems to make sense. And, I am not familiar with your speakers so perhaps your experiences will be different if you would try an 845 PP amp. Don't jump to conclusions just because I had a good experience with these.
The best thing is to listen to as many different types of amps as you can on your own speakers. I hope you have a few friendly dealers near you. Learn what works well or not: SET's, push pull, OTL, high or low power, whatever. And trying to get an idea what makes the difference can help you find your way through the wilderness.
I take my speakers to the dealer if he won't give anything on trial, which is not very practical with ESL's but well worth it in the end.
A while ago I borrowed a well-designed and well-built pair of 845 SET monoblocks from a befriended dealer. It had 300B's with anode choke as driver, and a 6SN7 first stage. On his own speakers they performed well. On my ESL57's they gave a very flat soundstage, same as you describe. Even the general direction of the instruments was very vague. The instrumental timbres were not natural; everything sounded alike.
Earlier I had borrowed a 300B SET (again with 6SN7 as first stage), and that gave the same results.
But: I have also listened to 845 push-pull monoblocks (Mingda MC300845-AB) on my speakers for a few hours. Apart from being a push-pull amp, the tube line-up was very much the same as the 845 SET's: Anode choked 300B as drivers, first stage with 6SN7, and no global feedback. The result was fantastic, with good depth and placement, and a very good rendering of the instrumental timbre. The most lively and involving sound quality I have experienced so far, while the other amps with exactly the same tubes were the biggest disappointments I ever had.
The only reason I didn' buy them is that I can't stand the sight, especially with their protection cages. Every now and then I still think of buying them and have something different made.
Same as other people have commented: It's not just the type of tube that means success or failure. I do have some idea what it is that makes the difference with my speakers. ESL57's are a reactive load, so the amps needs to be able to deliver a good peak current. That makes SET's not very good candidates, because they are low-powered amplifiers with a relatively high output impedance. The push-pull 845 amp had 70 Watts, so could still deliver enough peak current despite its high output impedance. My own Fisher SA16 (EL84 push-pull) has low power but with a very low output impedance, which also makes it work well. Until it runs out of steam in really big orchestra pieces, that is...
I can't really explain how this translates into things like soundstage, so it's not a proven fact but an idea that seems to make sense. And, I am not familiar with your speakers so perhaps your experiences will be different if you would try an 845 PP amp. Don't jump to conclusions just because I had a good experience with these.
The best thing is to listen to as many different types of amps as you can on your own speakers. I hope you have a few friendly dealers near you. Learn what works well or not: SET's, push pull, OTL, high or low power, whatever. And trying to get an idea what makes the difference can help you find your way through the wilderness.
I take my speakers to the dealer if he won't give anything on trial, which is not very practical with ESL's but well worth it in the end.