replacing a 12db filter for a 18db (or adding one to the system)

[joel]

Hi, 

I might be getting a Quasimidi 309, and i was really wondering if it is in ANY
way possible to add a 18db filter to the system , or.. alternatively
replace the
12db for a 18 db filter. (i REALLY _LOVE_ that original tb 18 db filter)
(the 309
has 303-type waveforms, so that's okay, but NO 18db filter) :(

anyone wanna help me out on this?