I have Sherbourn 5/1500A amp. I bought it about 3 years ago and it's been working very well - until recently. Not long time ago I started noticing that the sound "breaks up" or becomes uneven. It doesn't break completely, it's even hard to tell from the listening position what exactly is happening except you just feel that the sound is not right and is not full. If you come close to the speaker you'll hear that the volume slightly changes all the time. Some more facts: 1. It's not the pre/pro. I first noticed it happening with the old receiver, since then I replcaced it with a new one and a different model - it's doing the same thing. 2. It's not the source (CD/DVD). It does the same with calibration tones generated by the receiver. 3. Not the cables - I replaced the interconnects and checked speaker wires - everything is fine. 4. It can do it on all channels (!), but of course its most noticeable in the front. 5. It comes and goes. Sometimes it is very noticeable, sometimes less, sometimes it does not happen at all, sometimes it's the center channel, sometimes one of the fronts, sometimes two of them doing it. 6. It does not depend on the volume. 5. The #4 is very strange as the amplifier has monoblock design - each channel has its own power supply. The bad thing that although the amp still have warranty and I did buy it from an authorized dealer, I've lost the receipt. I wrote Sherbourn tech support about a month ago - never got an answer (have bad feeling about their tech support already.) Any idea what could it be? If not the amp, can it be main AC power? (hm... then why the same at low volumes and why it didn't do it before?) Speakers? (all 5 of Paradigm Studio?-don't think so) If the amp, why all channels in a monoblock amp? Some electrolyte capacitors are going bad? Anyway, any help will be appreciated.