Hi.
Not sure what your budget or knowledge is of modular synths is but if you are starting
from scratch here are a few ideas for the "Ultimate fat bass". You did say you had a "couple
"of racks empty and ready!

I would think you will need at least 2 VCO's . The A110 s are great but the A111s
are more stable pitch wise.
I wouldn't bother with the A113 . The A115 is good enough for what you are doing. Both
these modules need a pulse width sound to get the octaves though. The best deep dubby
bass sound comes from a simple sine wave. So you will need to mix the vcos before or
after filtering.The sine wave won't need to be filtered unless you are adding resonance to
the sound.
If you only wont one filter and have the budget I would get the A107. but A128 fixed
filter bank is great for fine tuning certain frequences.
You will need at least one LFO if you want your sound to have a bit of movement . You
can then control the pulse width modulation of the VCO . I would also recomend getting a
180 multiple connector for splitting the LFO around your modules.
Oh ..You will need a VCA 131 and an A140 ADSR. And probably a 138 mixer but you
could do with out that if you only have 2 VCOs

How are you getting your midi to the VCOs

I think buying Doepfer modules is like getting a Tattoo......Once you get one you have to
get another ...then another....It can be very addictive and if you have the budget you wont
be able to stop adding!

Good luck and have fun!
STUART