The unit is spec'd for 14VAC (yes, AC....rectifier is in the unit) 800mA supply. The one i am using is 12VAC/500mA. Can you say w/o reservation that is the reason for this ?
Raw math yields 11.2 Watts for the original. 6W for yours. It's likely your supply voltage droops below a threshold at times, causing distortion.
A unit does not necessarily need all the power specified on the label. It may not absolutely need that level of voltage, or that level of current. This makes it hard to be sure which approach you should take to boost your supply, whether adding 2V amplitude, or 300 mA.
The easiest way to proceed depends on what equipment you have at hand.
Conceivably your could take a 2VAC/500mA transformer and add it in series.
Or, take a 12VAC/300mA transformer and add it in parallel.
(Take care to keep waveforms in phase, of course.)
When internal circuitry rectifies AC, it implies it can also handle DC unchanged. So, do you have a 14VDC 800mA supply? Two 7VDC 800mA (in series)?
Etc.
You might test with one of the above arrangements, just to find out if greater voltage helps, or greater current.
There are converter circuits that turn 12VAC/500mA into a higher V at less A, or smaller V at greater A, but none that get more Watts than you start out with.