Hi guys. I am having trouble with my KRK VXT 6 i recently bought them and one of them is not working now. I just wake up in the morning and turned it on and it worked for few seconds then sound slightly went down and then vanished. but the speaker is still turned on power light is on its logo is glowing. anybody have an idea what's wrong with this? 1. When i turn it on it works for few seconds then sound goes down and down and then stopped. 2. I checked all steps in troubleshooting guide everything that say seems ok. 3. I swapped cables and everything but still same problem. regards..

SOURCE: guitar amp no sound or lighting

Ah yes: complete catastrophe. Usually when everything is not working, it is a single reason and when that single reason is fixed, so is the amp.

Based on what you tell me, I am going to say you have a bad power tube. And I will even go as far to say that you buy a set of tubes and play on them as long as you can. If so, then you not only burned up the tube, but weakened the power section and may have a slooow tube frier. I always tell customers: modern tubes have an effective life of less that 1200 or so hours and that varies. Replace the tubes once per year if you gig with the amp: period. Otherwise, guys like me (doing repairs) stay busy and the tube companies get rich selling tubes to amps with weakened power sections.
It sounds like you burned a power tube and blew a fuse. First, you need a new set of power tubes and some extra fuses (go to Radio Shack and set the value you need in SLOW BLOW fuses).
Then, replace all the power tubes. Don't worry about biasing yet, we are just seeing if there is a problem. Next, replace the fuse.
Turn on the amp and play on it at various volumes and settings. If all is well, take the amp to a tech and get it rebiased. If you can afford it, pay to have the grid/plate and other resistors changed so the power section will be like new (clean slate with new tubes). Your amp will love you for it.
Almost all of the amps I have worked on for performance problems (cant keep tubes to stay alive for very long) are directly related to end user use. When you use a tube until it blows it ALWAYS TAKES SOMETHING WITH THE TUBE WHEN IT GOES (like the power section components). The compents will be weakened and the tubes will 'wear' at different rates that can even move them out of the 20% tolerence they must be within to sound good. 99% of the time a board repair with a retube after a catastrophe fixes the amp until the next time it is 'run into the ground'. Tube amps are NOT invinceble: they are weak compared to solid state and expensive to own. But we love tubes because they sound great. I have solid state to knock around on, when when it counts, I play only with tubes. I have spent hundreds on good tubes because you do get what you pay for.

Hope this helps!
-mike

SOURCE: No sound from speaker, but works with external speaker plugged in

It sounds to me like you have blown the internal speaker.

SOURCE: My Line 6 Spider 3 75 Watt amp is having trouble.

Try this site:http://line6.com/community/community/support

SOURCE: i have a line-6 spider 30 amp that started humming

output caps. dont beat up your amp. crazy
yostamplifier.com

SOURCE: Amp volume drop with flabby bass sound.

1) can happen if the speaker is dying - voice coil defective or magnet needs recharging. happens sometimes with low quality speakers. 
2) faulty pre-amp section can reduce overall gain resulting in lower output volume.
3) for old amps dry solders in pre-amp & power amp section can have the same effect.
4) faulty coupling and filter capacitors can severely effect frequency response too.