Laney GH100L amp...weak/distorted output...Output tube plate voltage is 100V higher than the schematic states...any ideas?

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First, plug your guitar and cable into a different amp to make sure the problem is the amp. Then check to see if you have output from any headphone jack or effects loop. If so your problem is in the power amp section. If not, it's likely the preamp section. Whichever section it is, it will most likely need to go to the shop.

It sounds like a defective/incorrect AC-to-DC adapter may have caused your problem. These adapter come with different output voltages, AC or DC output and different amperage ratings. AC-to-DC adapters list the polarity of the DC voltage as it relates to the DC power plug and you must match both the devices voltage and power requirements. As for the amp...not working on batteries either, I would suspect output transistors or IC.

change out pre amp tubes

Sounds like output valve, if the amp is more than few hundred hours then suspect this for sure. I don't know much about this amp specifically but does the sound fade/return suddenly or over a few seconds? Try running it with the back off and fan on the valves etc. Then play at loudness where problem occurs and see if any improvement. Remember that at band practice you always play much louder than you think and put a lot of stress on it.

Distortion is not produced in the guitar. Distortion is generated in the amp by circuitry that increses the gain until the waveforms "flat top". This creates higher harmonics and a buzzy sound. If you are getting a very low output from your guitar it is possible that you don't have enough output to get the distortion in the amp to work. Other settings in the guitar add harmonics by mixing more than one pickup. This is not ditortion but adding these harmonics changes the sound immensly.
LOOK AT THIS SITE AND FIND A SCHEMATIC THAT MATCHES THE RESOURCES OF YOUR GUITAR... You might be able to figure where the wire goes... It may be a ground.

http://www.guitarelectronics.com/c=D65u46R1H9tko6CKpPLsjBhiJ/category/wiring_resources_guitar_wiring_diagrams/

OFTEN Roland has the opposite sense of normally closed contacts versus normally open. Test the switch with any continuity tester or an ohmmeter to see that the contacts work and come out to the switch. ALSO some amps require a TRS type connection (three wire plug) and others do not. From the schematic it appears that the Laney may REQUIRE a normally closed contact. There is also a pull switch involved that you MAY have to change the state of for operation of the pedal.

Here is the scheamtic for your amp:

http://elektrotanya.com/laney_aor100h_sch.jpg/download.html

Scroll down to the link at the page "Get Manual" and click to download. It is a JPG type file.

Here is the schematic:

http://elektrotanya.com/fender_vibrolux_6g11_schem.pdf/download.html

Check the quality of the DC bias as being free of ripple.across teh 25 mfd/50v cap filtering the bias.
If much ripple there, replace the cap. Also the .05 bypass on the swinger of the intensity control.

This should be an easy fix, but you might need an oscilloscope to diagnose it quickly... or just parallel caps across these to see if it kills the hum.

The main high voltage filter caps are also suspect as well as the bypass of power to the ealier inverter stage a 16mfd/450v

There are lethal voltages in this so use great caution when troubleshooting... the caps hold a nasty charge.

Realize that the number on the volume control is only relative... it means nothing regarding what the amp can produce. With a high level output guitar you can easily drive the amp into distortion at very low volume control level settings... what counts is the actual output sound volume and specifically the voltage output going to the speaker. Once the amp reaches saturation, any further input just pushes it into distortion or "flat topping".
An oscilloscope on the output will quickly show when amp starts to flat top.

The way you troubleshoot this is to divide and conquer. Run KNOWN clean signal into the "Power AMP IN" jack (note that it is a balanced TRS type input).

Verify the power amp downstream is clean. If not, then confine testing to power amp portion.

I would also check that the voltage across R66 is near correct IFpower amp portion is at fault.

I AM suspicious that the voltages on the scjhematic are AC peak to peak values rather than DC...

I think things in ovals are AC and rectangles are DC.