you can easily do the mod and try the effect...
the sound will be much brighter and bass heavy, in a single word, scooped.
there will be more gain and volume, but I suspect that a Dumble style amp without neg fbk will not sound so good.
NEG FBK is used to narrow the frq spectrum, flattens the amp response and tame the gain...if you took the neg fbk from an higher ohmage tap, such as 8 or 16, and you left unvaried the remain of the loop circuit, you are injecting a larger tension back to the PI.
The result is a darker and flatter sound, less gain and a bit less volume...
An interesting mod is to put a pot in series with the tap and a swtich to select between various taps.
with the pot you can esasily control the quantity of neg fbk going back into the circuit and with the switch you control where to take this fbk...
I've done this mod on my sloclone amp and it is very useful.