Hi, I recently bought a Roland Cube 80X. It has a very anoying sandy like sound when I use gain. I hae tried everything, plugging it into a different power socket, using a different guitar, (FEnder US strat and Tokay love rock) unplugging all wireless equipment etc. When I use a headphone the amp jut sounds great. I took it to the dealer were I bought it and no problem over there. Does anybody have an idea what could be going on???
Sandy? Like white noise, static, etc?

Might be a problem with the speaker if the headphones fix it.
You could be having some sort of magnetic interference. Is your amp right next to your computer moniter, a radio, or perhaps your cell phone?
Treble>Epiphone Prophecy EX - MXR micro Amp - MXR Blue Box - MXR Fullbore - MXR Noise Clamp - Vox AD30VT
Bass>Ibanez BTB505 - MXR Blowtorch - MXR D.I. - Peavey MaxBass 700 - Peavey TVX410