Unless you really know what you are doing, I would think about this a little more. If the head is still under warranty, it won't be after you drill into it, etc. However, if you want to do this, my guess is you should go to a tech/repair guy and have him/her check it out.
Another thing to consider. The EBS350 is quite 'scooped' in nature to begin with. Engaging that 'character' switch, which is similar to the Eden Enhance control set at about noon, you really do suck out all the mids. This can make your slap tone sound 'good' in a solo setting, but is a disaster in a mix, especially with a somewhat low wattage head like the EBS.
I am assuming you are a beginning player (no offense meant if you are not). Beginning 'slappers' (me included back in the day) often thought that the 'smiley face' EQ that results from controls like the character switch on the EBS heads was a good thing. It is not. If you really listen to the 'slap tone greats' like Marcus Miller, Alain Caron, Norm Stockton, etc., they have a TON of mids in their tone.
So... my point relative to your original post.... is to work on your slap technique without the character switch engaged, and work towards getting a good fingerstyle and slap tone with bassically the same tone settings, versus drilling into your amp
