i called my regular amp guy today (i'd avoided asking him at first just 'cause i'm thinking of working on it myself and i didn't him to know i might not be bringing him the business). he said a lack of low end doesn't sound like a cap or tube problem as when those go you tend to lose high end definition, clarity, and sparkle. he said it sounded more like an output transformer issue. does that ring true to anyone?
also, for what it's worth, it's the version with the contour knob and no mid switch. obviously, i've tried all yr standard troubleshooting including trying all inputs, a variety of guitars, using the amp w/ a different cab, etc. the problem is definitely in the box. as stated above, i can dime the bass and zero the treble and it still sounds very brittle and lifeless next to my twin RI (and that's a feat in and of itself).
No, it doesn't sound like an output transformer issue.
Click on this link to your schematic and follow along.
http://sunn.ampage.org/site/schematics/sceptre.gifScroll down until you can see the treble, bass and contour controls. Then look up until you see that dot connecting the vertical line from the treble control with a short section of horizontal line.
That component labeled 270PF connected between that dot and the treble control is a cap. This cap bypasses the high frequencies to the treble control.
To the left of the cap is a 56K resistor. This resistor feeds the signal to the bass and contour controls. Look at the arrow on the contour control and follow the line to the left and up and you will find a .022μF cap. This cap bypasses the higher frequencies to ground through the countour control. The contour control determines how much of the high frequencies are removed from the signal - the result is a bassy sound.
Notice there is another .022μF cap that has one end connected to both the bass and treble controls. This cap takes that bassy sound and couples it to the bass control which is acting sort of like a volume control. In the end, whatever bassy sound there is, will appear at the bottom of the treble control.
The treble control acts like a "blender". When you turn the control, that arrow in the diagram moves (pretend, ok?). When you turn the treble up, the arrow moves up and takes more of the high frequency signal through that 270pf cap. When you turn the treble down, the arrow moves down and takes more of the bassy sound from the bass control.
In a nutshell, that is normal operation. Now, what could go wrong to cause your symptoms?
Mentally, place that arrow in the position towards the bass control. Your symptom is that your sound is still way too bright. But in this position, the treble control should be getting most of that bassy sound. Obviously it isn't. What could cause that?
The most obvious is a lack of a connection between the treble and bass control. The wire connecting them could be broken, the treble pot could have an internal broken connection, the solder on either the treble or bass pot could be bad. Several possibilities.
Your other symptom is "lifeless" and lack of "umph" sound. I don't have a clue what you mean by that, but if it translates into a sound lacking bass and midrange, then this same cause could also result in that.
That's where I would start. If you have a meter that can measure resistance (ohms) you can check it pretty easy.
