I have the Project Tube Box SE, and have owned it a long time, for a Nottingham Horizon SE turntable. I have a Nait XS instead of an XS2. I have never heard a Stageline, unfortunately, but from what others have said about it I am sure the stageline would sound better than your tube box in your system, since you are also using other Naim gear. That being said, I have made significant improvements to my Tube Box SE simply by switching out the tubes, a less expensive upgrade with a major benefit. I would say that before you fork out significant money for a new phonostage, you consider switching the tubes in your Project. With your Tube Box you are getting some of that tube magic which pairs well with Naim in my experience. Also, instead of paying more for a phonostage you can perhaps save up for a TT upgrade.
In my Tubebox SE I started with the stock tubes, but they were not very good. If you have the stock tubes that came with the Tube Box, then I would definitely consider switching the tubes before buying a new phonostage. I listened to a lot of my Uncle's 12ax7 tubes he saved up (he is a major tube roller and uses all old-school low watt tube amp and preamp separates) and even compared to NOS tubes he had I found that the best sounding ones in my particular system were the Tong Sol tubes they started making more recently. It is like I had a brand new system (better clarity, detail, musicality, timbre, etc.). Then, later on, I got a pair of Electdol Chinese tubes, and I got even more of everything, particularly bass response. With the Elecdols I got yet another major upgrade in sound and I am very happy with them. I believe the company that now sells the same tubes that Elecdol made is called Northern Electric, and you can get a matched and balanced pair of those from the tube store for under $100. Totally worth the money to me. Below I link the Northern Electrics and the less expensive Tung-Sols.
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