I just change my JMlab806 to Lsim703 bookself, but I find that Lism703 is much more bass than 806.
Its seem that 806 sound is better in 2ch system. AM I correct ? Any cable or powercord , I can change to improve it.
or this 703 is like that. My room around 120ft. Amp is Yamaha RX3020-
I have found a warmer sound when running my Polks on a Yamaha. You may also want to check that the enhancer feature on the receiver is turned off. The default is on and in general I don't like how it makes my 703's sound. You may also want to change the eq to flat or through. You can also adjust the base down slightly on the receiver. I have also found that a factory reset and rerunning YPAO.
Generally speaking, JMlab speakers tend to be leaner and more detailed sounding than Polk's LSi series, which have a nice warm sound most of the time. Has to do with the metal inverted dome tweeter on the JMlab speakers. Personally, I find the Polks to be more pleasant for 2 channel use and JMlabs I've experienced better for home theater. They can be kind of shrill to my ears if paired with brighter electronics, which I find Yamaha receivers to be. Personal preference thing though. I wouldn't recommend trying to tune things with cables or power cords in this case when the root of the problem is either you prefer the leaner sound of the JMlab speakers or the receiver you're using doesn't pair the way with your LSi's the way you'd like. Running the eq in the receiver may be the best bet (and cheap too).
To the original poster - did you re-run the Yamaha YPOA after swapping in the LSiM703s?