If your tv has a fixed line out then you cannot use the tv remote but if like some tv (mine for example) it has the option of a variable line out then you can use the tv remote, it maybe worth lookijng if there is the option to do this via the tv's setup menu.
I do not know your amp but does it not have an aux input, normally the AV input is simply a rebadging of the pre amp IN input and therefore bypasses the amps volume control whereas the other line ins go through the amps preamp, and therefore volume control, stage so you should use one of these inputs if available.
Or as was suggested use a freeview/sky/freesat type box as these have variable volume audio outputs (well mine do)
Tommo