Diff bet Pass X350 and 600 insound quality/

Are both balanced?Feedback design?Low freq resolution?
They are both cut from the same cloth. You should make your decision based upon your speaker and budget. I had the x350 on my Thiel 7.2s and then moved to the x600. The Thiels really opened up and had better lower frequency imaging and clarity, but this is because the speaker requires power. With this much power on a full-range speaker, the sound is quite amazing.

With Cello Stradivari Master speakers, X600 presents a much bigger (more open) sound stage and better mid and lower frequency imaging and clarity again X350. I agree with Rtn1 that if the speakers don't need that much power, the differences might be not quite noticeable. However, for power eater speakers, X600 will make a huge jump.