[Amps] SB-230 drive VS power output

[K4MT]

I have been fighting with my SB-230. Initially I was getting 500 watts CW key down with about 85 watts drive and over 600 on my peak reading meter on SSB. 
I slowly lost gain and finally I was only able to get about 300-400 key down and about 450 ssb peak. I decided the 8873 may have died. 
Today I took the tube out and cleaned the Beryllium block and heatsink, and also cleaned a black badly tarnished grid ring and replaced both bypass caps. When I put it together with new thermal grease etc, I now am back to 500 watts cw and over 600 peak on SSb. This is with about 85 watts drive. Don't know what fixed it!?
Is the normal for these amps as I can find no specs listing output power vs drive level.  I noticed at 100 watts drive there is noticeable flat topping but at 70-85 watts I get a nice clean scope display.