Try to figure out if you like more direct sound by listening to your KH 310 with about 1.5m distance if you like it better than in 2m distance you like a higher amount of direct sound. In this case the KH410 provides the quality of the KH310 at your listening distance of 2m plus the benefits of a higher listening distance.Only thing is not really clear for me is the directivity issue.I beg your pardon but I don't have a technical background.
If you choose the KH420 I would recommend dirac live as room correction.
I think the best investment would be room acoustics and correction however you implement it.
Also very deep bass is a good improvement. Even with the KH420 I would add multiple real subwoofer which can playback 16Hz. This will improve the sound a lot with some recordings.
The biggest Genelec One is objectively better than the KH420 and I would consider these. Kef R11, Me Geithain RL 921 are also interesting. If you will definitely go the speaker path.