I demoed the two into an NDX2 a while back. I went for the 555 in the end. They were both very similar in adding much more resolution. They have different sound signatures though. I’d heard some of the new classic stuff and I thought the 300 sounded similar, “a cleaner sound” etc. The 555 made the NDX2 better but kept that “natural or live” sound. I was also thinking about synergy. My experience with Naim is that mixing different ranges doesn’t always work well.