Understand that the QP400 is 300ohms. Which amp you using to drive the QP400 ?
How's the SQ compare to the GMP435S (32ohm) ?
Just got the QP400 in.
I think this headphone will suit your needs much better.
Given you love the GMP 450 PRO except for the fact that it is indeed closed, the QP400 sounds perfect - the driver is identical, the clamping force is slightly greater, and the soundstage is quite large.
To be honest, it lost a little resolution and speed, but the open design really livens up the vocal ranges.
More on this as I A/B with more tracks.
My roomate and I plan on posting something, as mentioned before.