see the problem is a bass knob can and usually does add distortion and drives the amp into clipping which can damage your subs. that's why people avoid using bass boost circuits on amplifiers.
as for 300 x 2, does he need 300x2 @ 2 ohms or 4? can he use a 600w mono amp? 2ch amps are usually class AB and more costly per watt than a class D mono amp, and both work fine for subs, where the class D is cheaper, runs cooler, and takes less room as well as draws less current to make the same power.