You may well have found some transistors that sound better for this circuit than others.
  However, you can not jump to the conclusion that the frequency spec. is what is responsible for the difference.
  Roly has shown how this is the case.  There are many other aspects of transistors that could be responsible for the difference you hear.
  It could also just be a coincidence.  You would need to try many samples of each type of transistor to reach any kind of conclusion.

  One of the simplest things to try is a spare cord between the FX send and return jacks (or preamp out & pwr amp in).
  These jacks have internal switches that route the signal through.  They often cause problems like this and are a great place to begin trouble shooting.
  If that doesn't help, try plugging your guitar straight in to the FX return or pwr amp in jack.  If the problem stops, it is likely you have a preamp problem.  If the problem continues, you have isolated it to the power amp or speaker.
  Alternatively, you can run the FX send/preamp out to another amp and see if the problem is still there.

  Those views are mostly bots.
Glad to hear you are paying attention to safety.  For solid state stuff we don't usually worry about discharging, but it is good practice to check voltages to make sure things ARE discharged.  With tube amps it is a big concern due to the high voltages and circuits that often don't self-discharge as easily as solid state stuff.
  Sounds like you have a pretty good idea what to do and what are likely suspects.
Also check generally for other bad solder connections.  If you have a good idea of what bad solder joints look like, a visual inspection will go a long way.  Pots and jacks are also prone to solder problems, as well as "hot spots" like power supply diodes and resistors where you may see browning of the board due to heat.
  Pay special attention to the dis-assembly and re-assembly process.  Take pictures and notes if you can.  Pay attention to wire routing (dress) and connectors as well as the heatsink assembly for the output transistors.  It is very easy to break wire connections when getting the board in and out of these amps, or flipping it over for access.
  Connectors are another item to check for bad solder.

Quote from: pudge211the H1- and H1+ voltages are 20.3 respectively

Please measure with your black probe connected to ground, use red probe to connect to H1-
Be sure your meter is set to DC volts.
Specify whether your reading shows negative (-) or positive (+)

  Thank you JP !!!

  Try it with a patch cord between the FX send and return jacks.

  I tried to upload it here, it won't accept as it says it already exists.  Yet a search of this site can't find it?  :loco
Here's the direct link to the one Matthias mentioned, there are 2 halves and a parts list:
http://music-electronics-forum.com/t20702/#post171291

  Are you stuck on the switchable output power feature?  If so, I think you are stuck with this transformer.
  Otherwise, why not look into other tweed Champ (5F1) kits.  That's basically what this is, and there should be plenty to choose from stateside.
  This is a very simple circuit so they should all be roughly equal ease of building.

Doc, there was a thread over at MEF that went into detail about the ground scheme for this mixer.  JPB made up a pdf of the ground info, but there was more said about it later so I'll just link the thread instead of posting the pdf.
The ground info starts here and carries on further down:
http://music-electronics-forum.com/t26867/#post340734

  Certainly a very strange set of symptoms.  As all channels but 3 have the same problem, normally you would suspect something common like low voltage power supply, but ch.3 working throws a wrench into the works.  I would suggest you leave the ch.3 issue till you have the other channels sorted.
Find out if the FCL's are lighting when they should, if so, the signal at that point should be correct and the problem is downstream somewhere.
Levels for FCL threshold are shown on page 129 up by where it says "mix".

  And above all,
DON'T PANIC !!!
(and don't forget your towel)

 I've seen it said there should be 25mV between TP301 and 302.  I've also seen it said 2.5mV, however the early versions were mosfet output devices and I believe the 2.5mV was for the mosfet version.
  Can anyone confirm or opine whether that sounds likely?

  The fact that it responds to you smacking it means it is likely an intermittent connection.
  As Roly mentioned, this rules out defects of the capacitors themselves.  However, their solder connections could cause the problem you have.  Often resoldering the filter caps corrects this type of problem.

  Clipping of the power amp will occur when the amp achieves it's full power output.  Where the volume control is set does not completely control it, it also depends greatly upon the level of the input signal.
  A very weak pickup will allow a higher amp volume setting, but a "hot" pickup will drive the amp to clipping at a lower amp volume setting.
  Many amps will clip at around 4 on the volume control.  Turning down the volume control on the guitar itself will allow higher amp volume settings.

And in case you are unaware, the 7815 and 7915 regulators have their inputs and outputs in different positions.