Spaceplane speeds on Kerbin- beating 1400m/s

I'm doing my best to launch an SSTO into orbit. Currently, my primary objective is to replace my LVT-30 with an LV-909 so that I can reap the d/v rewards. Unfortunately that means pushing as high and as fast with the airbreathing engines as possible.

I seem to have hit a brick wall at 1400 horizontal meters per second. No matter what I do, my craft won't beat that speed. Levelling off soon enough to have enough air to push faster means too much drag and the plane explodes.

Is it even possible to break this barrier (~>Mach 4) in Kerbin's atmosphere? If so, how can I re-design my craft to achieve it?

I added More Engines™ but only managed to increase to 1456m/s. I've managed to hold that speed for several minutes now at about 10km alt but can't break it- the craft doesn't explode but no longer accelerates.