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As far as volume/tube issues....if you have a spare 12ax7 sub it thru all the positions ands see if it cures the issue.
As well, check out the pictures in the "you might need a cap job if" thread....check the bottoms of the caps for obvious dimples and bulges. Burnt resistors may be a culprit. As Dave said,swap the power tubes if you have a set handy,or cheap.
Ive also seen defective volume pots.....could be a number of things.
Go with the tubes first tho......post the results!!!
Cheers

Hi ,

I'll check all of these solution , but i'm on a very nice pair of blue TESLA EL34 NOS(1937) ,so i must wait a little more

I open my amp again , and i put some Groove tubes GT12AX7M in V1 V2 and V5 , and i haven't heard a big change in my sound...

It's very strange because when i put my guitar into the normal channel 'Hi' input , Input volume at noon , and master volume at 9'o clock , my guitar sound is practically inaudible , but with the master at noon i have a nice clean tone with "normal" volume.

And after that , i saw my caps...

http://blindj.free.fr/Images/Hiwatt%20Cap/

I think i need to re-cap

it is certainly due for a recap.

Have you played through this amp for very long? Did it have more break-up and volume in the past?

Most of my DR504's don't break-up much until they are pretty loud.

I would recap it and try new power tubes then see how it sounds.

I bought this amp in 2007 , and haven't played it a lot (It was my band amp) , and i have never try to break-up this earthquake machine

but today , i shunt my Normal input with bright input , turn both input volume to max , and play one chord with the master at noon... loud , it was... loud (on my 2x12" Reeves with PV speakers)