I bought a brand new Roland Micro Cube Bass RX a few days ago for quiet practice. Didn't think to try one out in a shop on my newly acquired Prosebass 4 string headless, and just ordered it sight unseen from the Normans website. First off, the manual states that only alkaline type AA cells can be used, not rechargeables, so could work out quite expensive to run. But the big problem for me was when I tried using the amp with the supplied 9v 'wall-wart' mains adapter; I immediately got a massive hum that increased alarmingly when a note was played. This wasn't present when using the AA battery option, so I tried moving the amp to different parts of the house, and even into a detached garage to avoid any possible interference, but with no improvement.
In desperation I asked Paul Rose for advice and learned from him that my bass had been wired with no bridge earthing, so as to avoid electrocution should a powerful amp ever malfunction, and this arrangement was causing an earth imbalance when the wall wart was used. Straight mains powered amps and straight battery powered would be ok though.
I returned the amp and got a full refund and will use the 'Earwig' headphone amp I've just bought instead. However for acoustic jamming, busking etc this little amp would be ideal if you're happy feeding it a diet of Duracells, and don't have a bass with a non-earthed bridge for use when mains powered.
Jerry