What version of the switch is this? For 2 pickups or 3?
If it’s for 2 pickups, is it the standard one or the Lee Sklar one?
If it’s for 3 pickups, is it the standard one or the D-Bird one?
In a different price category but the best standalone headphone amp of all time for bass is the Phil Jones BigHead. If you don’t want to pay full retail price, they do pop up once in a blue moon on the used market for around £100-ish.
I used to own 3 of these in various configurations. Now down to 1 but if it weren’t for Covid I’d still own all 3. Frankly the best P basses I’ve ever played. GLWTS sir!