[Amps] AL-811H low output

[Hugh]

I recently acquired an Ameritron AL-811H  HF amp.
The tubes in it were in rough shape so I replaced all 4 of them with a 
new set of Chinese tubes.
The best I can get is just over 400w carrier with 75w drive on most 
bands.  The HV reads about 1650V unkeyed and sags to about 1500V when 
keyed. I read somewhere that the suppressor resistors can go high if 
overheated, causing low output but I checked them and they all read very 
close to 100 ohms as marked. 
Are there any other known problems with these amps that can cause low 
output ?  I'm all ears to any other ideas before I go ripping it apart.

Tnx, Hugh VA3TO