Ampeg SVT VR sometimes quiet

[Wild Steve]

Just playing with an SVT VR and noticed something odd. I turn it on and with the volume at about 10 o'clock (so about 2 out of 10) it's trying to blow the house up in traditional SVT style. I'll then stick it in Standby or maybe switch it off then come back hlaf an hour later and at the same volume it's really quiet. If I then turn it up to half way it's putting out a reasonable volume that'd be fine for a mellow home practice - yet in normal times at half volume it would be virtually blowing the house down!
This has happened 3 times now, but always taken me by surprise so I've not been able to recall exactly what I've done so I establish a pattern. Anyone ideas about what might be up?
The other thing I did notice was that when just out of the box it had lots of little crackles and spits which seemed to be due to the preamp tubes as they didn't change with adjusting the volume, but they've gone now - does it just take a few hours to "break in" new tubes?
Steve

[craig.p]

Stab in the dark: Try reseating every tube. Leave them in their original locations. Could be you have one or more oxidized or loose sockets.

If that amp has an effects loop, bridge it with an instrument cable to eliminate a possibly dirty/corroded NC contact in the effects return jack.

Crackles and spits when new is not good. Again, possibly loose/bad tube socket, maybe a bad tube, maybe a loose component, bad solder joint somewhere . . .