Clip light is on even when no input and when bass is plugged in input low volume and distortion at speaker output

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.

You can put just about any instrument through any amp. The difference in sound will be whether or not that amp was designed for the particular frequencies of that instrument. Also whether or not the speakers you are using are designed for those frequencies. When playing bass you need speakers designed to carry low frequencies (40 - 400 hz); mid-ranges and tweeters don't cut it. Set your instrument volume on the guitar at 70 - 90 % to get the most signal from the instrument. Set output volume on the amp at 50 - 70 %, then adjust gain on the amp to get required sound level. You're not likely to blow the amp, but you could blow the speakers if the amp delivers more power than the speakers can handle. With a micro amp check the output rating. It's possible you may not have enough power to get the sound you want and as you turn up the gain the sound will distort.

You MAY be driving it too hard. If you are sending large bass pulses like from a drum machine to this, it may be overdriven. Check if the clip light is coming on. If it is coming on, back down the volume. If you can't live with the volume at a level that doesn't clip, then you need to get a bigger speaker. This is a 12 inch which is really not suitable for running drums through. Good for vocals and some instruments. When you run up to clipping level the gain is cut in the electronics to protect the unit and that results in a distortion or fuzzie sounds.

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.

First of all confirm if the speaker impedance is not lower then the recommended Ohms at the back of the Mixer speaker socket.You can use an 8 Ohms speaker on a 4 Ohms amplifier but not vise versa .
Next see if the fan on the mixer works. If not the Amp is getting very hot and this will lead to unwanted distortion. You can confirm if its a heat related issue by playing the mixer at low volumes for more than an hour and see if it distorts.

A common reason for 'distortion' with all bass amps can be either that the screws holding the speaker unit to the cabinet have worked loose or that there has been a build up of dust particles in the corrugations around the edge of the speaker cone, which rattle especially on the lower notes.
The solution is simple - remove the front grille and check the screws are tight. Use a vacuum cleaner to remove dust from the rim of the speaker cone.

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.

The output stage has a problem. From your description, I'd guess that the high voltage supply has failed. The low volume and distortion suggests that you are trying to power the speakers from the driver section rather than the outputs. There is a resistor that fails in some of this series. If I remember correctly it is either a 2K or 3K resistor 10 watts in size. If you have no experience with repairs, I'd seek a service shop that is familiar with these units. The part runs between $5-$10.

Dan

You can reach the limiting IRRESPECTIVE of where the volume control is set when you have a guitar or other source that has HIGH output.

Guitars that have battery in them with internal amplifiers are an example of a high output guitar. Plain inductive pickups may require the volume control to be set at say 70% to reach limiting.

Please NOTE that the volume control is NOT linear, but is an "audio taper".

Limiting means you are probably driving it too hard. Many learn the hard way when speakers and thee power amp fail.

When the cone is driven too far beyond the pole pieces of the magnet, then the IMPEDANCE of the speaker becomes relatively low and can damage the power amp. The limiter tries to prevent this by sensing the current and or voltage and will back off the gain internally. This causes distortion when it happens.