Impossible. The amp can't do it, and neither can the tubes. Completely impossible.
The SB-200 was rated "1 kW DC input power," not 1 kW output power. Big difference. Amplifiers only started being rated for output power when the rules changed to allow 1500W PEP as the legal limit, in the late seventies. Prior to that, all equipment was rated for input power, as that was the only power reference in Part 97.
The SB-200 should indeed provide 600W PEP output. Possibly 650W PEP with a good, stiff AC line and new tubes.
What if soy milk is just regular milk introducing itself in Spanish?