300 mA seems rather low - it's supposed to be 2000 mA. Tantronics (.co.uk) have a 12V 2000 mA switching adaptor that fits the t-amp just fine. Map.lins do a 13.8V transformer but from the picture of it I couldn't see how it would connect to a t-amp, so I went with the 12V switching adaptor. I've been using that for about a month without problems.
Volume wise, if I have windows and wave volume on 50%, the t-amp needs to be at about 40% (11 o'clock position) to give good volume (with 8 ohm speakers).
The t-amp definitely has some advantages - it's small, light, uses little power and doesn't get hot, plus it does sound good. I tested mine a lot versus my Marantz PM66-SE, which is a respected £230 amp, if a little dated now, and the t-amp held it's own in most music (particularly rock and pop). You probably would have to spend over £100 on a used integrated amp to outperform it.