If the master output lights go above green (into yellow) the sound becomes slightly distorted even at low speaker levels. i have cleaned the output jacks but it is still distorted

Guitar amps have preamp sections (think unpowered audio mixer) and power amp sections (Crown rack-mount amp). An excellent way to isolate audio problems if amp is still producing audio is to check the preamp out (line out). Preamp output distorted = preamp failure. Connect MP3 player to "Power Amp In"...sound distorted = power amp failure. In your case the latter is the case as your "line out" is clean. Also test each speaker for clear sound as speaker cone damage from being overpowered is fairly common. At 300w. I'll assume the failure was not from cranking the volume to loud but a shorted speaker or speaker wiring could zap the power amp in an instant.

Try connecting them to another amplifier...you can use the speaker output of a guitar amp as a quick test. If the amp (and mixer) check OK, it sounds like you are about to become good friends with your local speaker reconing shop.

Remember...Distorted Audio Can Kill Speakers even at low volumes. It happens because sine waves become square waves. Google it for additional information.

You need to check the actual signal levels to make sure they are within the specs for the unit. The mic input is low impedance. Your mic should be like a Shure SM58 or equivalent. The output is fairly high... up to +14DB which will require you to turn down the trims on a mixer this runs into or set a -20Db pad at the input. I suspect your distortion is from the device output to the mixer input. It is VERY important that the mic does NOT "hear" the speakers as the feedback loop resulting will be mass distortion. The guitar should also not "hear" the mains.

One half of the power amp has failed. You can damage the speaker... do NOT use this until repaired.

Yellow should be the positive. Try plugging in and reversing. The louder of the two is the one in phase and the correct wiring. If the speaker is pushing out when playing , it is in phase and correct also.
The distortion problem could be a speaker going out. It may not be totally blown, but may be verging.
Holding your hand out as wide as you can push against the speaker lightly. If there is a rubbing or scratching sound, the speaker could be damaged.If it moves freely, and there is no scratching its okay.
Another possibility is the clean channel may have a channel volume set to high. Make sure the clean channel volume is set lower than the master volume otherwise you can get distortion on that channel too.
Good Luck.

Proper level setting of the mixer is important. HOWEVER if the clipping is occuring at the speakers the only possible problem MIGHT be that you have a supersonic feedback that is saturating the speakers above your hearing range, Be sure you don't boost the highs too much with the EQ as this can cause the feedback that you can't hear. Also make sure that you power the speakers from the SAME receptacle as the mixer, even if it means running an extension cord to bring power to the mixer. This is to avoid a low frequency hum and common mode distortion/damage. A low frequency hum could cause the clipping.
It would be a good idea to get a sound meter to check the sound level. You should be able to reach 85 Db from this system without clipping. If you need more than that, you MAY need more speakers if the band instruments are too loud. Also if the band has amps that get into the vocal microphones that adds to the clipping level... make sure the mics don't "hear" the band instruments. Make sure your speakers are toward the audience from the mics to avoid the supersonic feedback problem. If the vocalists can't hear themselves with that configuration you need to set up seperate stage monitors.

There are some pretty good videos on YouTube about proper mixer level setup.

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.

Just because tubes light up does NOT mean they are good or working... While heaters not lit are an indication of failure, teh reverse is not necessarily true.
Time to troubleshoot this... Plug a set of headphones into teh preamp out. See if audio there is clean... it will be low level using headphones and ONLY from one side as the output is mono and your headphones are stereo usually... If audio is clean here and not muffled, then problem is in power amp section else in the early stages of the unit.
Report back the results of this test...

Check following things:

1. The master volume control on the SR16 is at least half way up. Turn down the amp as needed. Running very low output of the SR16 and high amp gain is sure to get distortion.

2. Check power adapter is the original and is producing correct voltage.

3. Check that audio cable is high quality instrument cable and not consumer or speaker cable.

This has the earmarks of a failed power adapter. If the voltage is low, everything will saturate. WARNING: You could do damage to the electronics if you try to drive high levels through the unit to compensate for low output from unit.

Check the voltage from your adapter. Check it UNDER LOAD as it may collapse when connected to the unit.