IS, I have two rooms but mixed match the amps and speakers from time to time. To my ear, LS50 matched better with Leben. Probably because of the power.
BA, LM 219ia is a great amp. Very seductive with great center feel. Had great outcome when I upgraded stock tubes with Elrog 300b and 845 but that is another 3k upgrade. I cannot compare to straight 300b (don't have much experience there). Main reason I bought LM 219ia is that I wanted more than 8-10 W to drive my speakers. Only problem with my LM was mechanical hum from transformer. You cannot hear it once you start music but it bothered me. Otherwise, I kept the amp. Now, I replaced LM with Shindo. I read some other LM owners complain about the hum. Overall, LM makes very good amps and they are great value. Just make sure that you don't get the one with hum.