try not to put too much power to it...
unless your custom sub has some real good suspension...
my theory is that quarter wave tlines have very little backpressure... thats why you cant throw too much power into it...
i would say a little below RMS with clean/uncliped power and you should be fine...
if i were you i would do do 1000w RMS, mayb 1200w max...
i burned my lvl 5 in a tline cuz i was being stupid...