[Amps] Requesting opinions on a Drake L-4b's tubes

[A Lawrence Fibich]

I just acquired this Drake L-4b as the third owner and no nothing about its history. It puts out 400 watts with recommended Plate and grid currents (.56 amps, and 225Ma respectively). If overdriven (.8 amps; 100ma) it will do 600 watts. At 'idle/resting' should be 0's but shows .1 plate amps & 75 ma grid current- which I think shows a tube problem.
Now my request for an opinion, what do you suggest:  
1. I got 2 unopened tube boxes from Drake - thinking to simply replace/use them and see what happens.
2. Surprise - 1 is the 3-500G- (Graphite plate) - the other turns out to be a 3-500Z (old style) . 3. The unit has 2 (original/old) - 3-500Z's in it and really no way to test them4.  So? what to do?5. Live with it, and leave the 2 in the unit and live with 400 watts and? or- maybe overdrive them and go for 600 watts? ? how long do you think they would last?6. Use the 'new' 3-500Z with one of the 'old' 3-500Z's?7. Use the 'new' 3-500G (graphite) with the 'new' 3-500Z?8. Use the 'new' 3-500Z with one of the 'original/old' 3-500Z? 
Of course, the finals in the Amp are push pull configuration -2k PEP & 1K CW input watts. 
Thanks for ALL your suggestions/recommendations.  73Larry F.W6EF