I had the A23+ as well with my Vinnie Rossi LIO and had Amperex 7308 white labels as well as green labels and thought it was quite good. I never felt the A23+ was underwhelming at all. As far as the A21 , it just sounds better with my "new"setup using the Mac 2300. It was a tough call but it did sound more to my liking, a bit smoother than the 400 but that's not a knock, just the way I listen to my music these days. I did not try the 400 nor the A21 with the LIO. Perhaps down the road but right now I am committed..lol..to the Mac. The Mac with the new Tung Sol 12x7 gold pins is quite beautiful sounding, much better than the original 12x7 's. I sold my Genalex 12x7 Gold Lions a while ago when I sold my Mac C220. The only limitations with the Mac 2300 is it is hard to swap tubes compared to the C220 and the ease of the VR LIO. In your set up you might hear things differently and the 400 is the better fit. Cables as well as speakers make a difference as well. ..not to mention I will be 70 at the end of the year and I KNOW my hearing has changed and not for the better. As far as comparison between the A21 and the A23+...build quality is outstanding now on the 23+, much better than the older one and better in some ways than the original A21. The new 21+ has taken the build quality to a higher level as well along the lines of the JC5. Sound wise the original 21 I have is better overall... sound stage, bass, everything over the 23+, it's just that good. Not so when comparing the 21 to the 400..both are excellent and give so much great sound and detail. Both are quality built . So it comes down to a gut decision. If I could keep both..I would...this was a tough tough decision.