You can add a passive tone control yourself. You'll need to drill for another pot, or swap one of the current pots to a stacked pot. But there's definitely no reason to replace the preamp or do anything else drastic.
You'll use the same basic circuit - a pot in series with a cap between hot and ground. The same exact thing you'd use as a tone control on a passive bass. There are a couple places you can add it in order to get different functionality.
If you want it to be in the circuit all the time (regardless of if you're in active or passive mode) and you want the pot to load the pickups (which will warm up the tone all the time - regardless of if you're in passive or active mode, and regardless of the tone control's position), you can tap in to the output from the blend control. Use typical passive bass values, i.e. a 250k or 500k pot and rougly .047uF cap.
If you want it to be in the circuit all the time but not load the pots, put it directly on the output wire to the jack. Use typical active pickup component values, i.e. 25k or 50k pot and .1uF cap.
If you want it to load the pickups and only be part of the circuit in passive mode, find the terminals on the active/passive switch and look for a pair that are bridged together - that'll be the terminals that are selected in passive mode. Connect the pot to the jumper between those terminals. Use typical passive values, i.e. 250k or 500k pot and .047uF cap.
This is one of my favorite mods on basses with active EQ preamps. If I want the bass to have a passive-like tone, even when in active mode, I use the first option. If I want the bass to "sound active" I use the second option. I generally don't use the third option much. Sometimes I'll use a tiny trim pot, and use it to set a baseline tone, then tuck it inside the cavity as a set-and-forget way to trim the tone of the bass.