A guitar body is not a Helmholtz resonator. In a Helmholtz resonator the body is non-resonant and it is the air in it that resonates. Also the Helmholtz calculations only work if you assume a stiff plug of air, ie a long thin neck to the bottle. A guitar is not a long thin neck of air into the body. Thiele-Small calculations don't work for guitar bodies either.   

In a guitar, it is mostly (not entirely, mostly) the top or face of the guitar that resonates - and mostly the lower bout, not the upper bout, but the upper bout does do a bit of the work. The shape of a cutaway is plainly and obviously stiffer than a non-cutaway - ALL OTHER THINGS BEING EQUAL. Therefore less thud and less resonance.