[Dan Banquer]

"When probes are held up in air, meter readings are similar, but not the same.  They are jumping around in the several hundred mV range.  When testing the ouputs, readings were unstable but didn't go beyond + or - 30mV.  When shorted together, reading goes to zero."

If probes are held up in the air and the meter is on AC it is likely that the meter is picking up 60 Hz from AC line. The use of the Hz function on your meter will confirm this or not. If you are on DC then you are probably picking up a very low frequency (subsonic) as the DC section of the meter will react to this subsonic frequency as a varying DC level. If the meter is on DC and correctly hooked up to the output of the pre amp and the readings are at +\- 30 mV then you probably have a low frequency oscillation and the amplitude will be far greater the +\- 30 mV as the meter is not capable of responding to low frequency variations accurately. In addition I have observed tube pre amps with worn out tubes to fluctuate at idle at subsonic frequencies. If this indeed the case then your direct coupled power amp will amplify this and send it right to your woofers. Please note that many meters do not perform accurately below 12  to 15 Hz.

It appears a scope is required here to determine the exact nature of the problem.
Regards;
 Dan Banquer

[JoshK]

That makes a lot of sense.  I had a few guesses, but they weren't worth much given my lack of experience.  I thought it might be something due to the heater supply ripple modulating the audio ground. 

Can I take this one step forward?  How would you hook up your scope to measure this?  Just put the probes on the RCA output jack and set the scale for a range where +/-30mV is seen clearly and then dial in the frequency until you see the oscillation?

[Dan Banquer]

That makes a lot of sense.  I had a few guesses, but they weren't worth much given my lack of experience.  I thought it might be something due to the heater supply ripple modulating the audio ground. 

Can I take this one step forward?  How would you hook up your scope to measure this?  Just put the probes on the RCA output jack and set the scale for a range where +/-30mV is seen clearly and then dial in the frequency until you see the oscillation?

I would connect the scope probe to the pre amp output and gound. Set the scope at the lowest voltage setting and the 10 millisecond time base, and go from there. Just as a basic primer on scopes: The x axis is time the Y axis is amplitude.
Hope this helps;
   d.b.

[JoshK]

Yeah, that is what I was trying to say. 

[toobluvr]

Thanks Dan!

[*Scotty*]

Josh, a scope is a precision voltmeter, one calibrates the scope with a precision voltage reference and away you go. You know the exact magnitude of any voltage displayed on the scope. This give a quantitative value to a problem you are evaluating.
Very cool IMO.
Scotty