Good question… You’ll get 20 opinions but I’ll tell you what I did.
I have not yet purchased a CP-251 but still may someday.
I had a Little Phatty with the CV Out mod done to it and my first Fogger was the 105M. I’m a Bass player and the MuRF appealed to me for 2 reasons. First, it had an array of filters that could be messed with, it could slave to a Midi clock or to the Phatty’s LFO, and the patterns gave me a good palette of colors to play my other instruments against.
One of the nice things about the LP that many people don’t get is that it has an Arpeggiator built in and the combination of that + the MuRF makes it alot of fun to mess with.
A little talked about feature of the MuRF is the LFO/Sweep capability which is a switch that will affect a phase/modulation between left and right speaker like a leslie in LFO mode or will sweep/shift filters while in sweep mode (w/switch off). This is most useful with pattern “0” which does not use any automation. It is otherwise difficult to discern with Patterns running since every other filter is assigned to L or R channels when run in Stereo).
The other good thing about the MuRF is that it can tame other inputs (as you’ll see in a sec when I talk about my 2nd Fogger purchase!). In this way, it acts as a Filter but with a very complex envelope setup (difficult for me to explain; I’m no expert).
My 2nd MF purchase was a FreqBox. I’ve seen all of the online demos and have not been particularly inspired, but one that I caught showed a good enough range of sonic possibilities to make me take the plunge and as you’ll hear mentioned, you can use it to add a 3rd OSC to a LP or a 4th OSC to a Voyager family kbd. To me the Freq is [at least] 4 Pedals in one, it’s an OSC as mentioned, it’s an Envelope follower that can be used to trigger and modify behavior on just about anything [see examples below for starters], it’s a drone, and of course it’s a wild-ass (technical term) distortion pedal which I’ve yet to tame.
So starting backwards…
a. It’s got such a varied array of controls which you likely need a PhD in Moog to understand. Though Bob was not part of the development of this pedal, he can be heard saying [on the Moog DVD] that he likes to give people as many controls as possible with as many options per control as possible [or something like that]. In this spirit, the Freq does just that. I don’t know jack about Phase Distortion, Frequency Modulation or how Envelopes work in this context just yet but believe you me, between varying the waveform in this standard mode and the other controls, the right settings when applied to the knobs will create a wild time. And as another poster once said, it’s most useful when used with a Filter [either MuRF as I mentioned above, with MF-101, or while using the Envelope follower to control your Voyager’s filter [see letter c. below]. In this case, you just run a TRS cable from ENV OUT to Filter in, then pump your 105 Audio Out into the Voyager Audio In, quiet all other OSCs, turn your kb gate on, SUSTAIN of the Volume envelope on full and let 'er rip.
b. As Drone, you set SYNC to “off”, then tune the Freq. to the frequency of the key you are playing it. While not particularly exciting, you can use the Freq as a pedal point for something played in that key or an adjacent harmonic.
c. Env Follower - as touched upon in a. above, you can route your guitar into the “in” of the Freq. then as mentioned above, use the Voyager’s filter not to control the output of the Freq, but purely to filter the instrument signal via the Voyager. (poor man’s 101; get it… poor man!)
d. As above but instead of driving a Filter ENV, use it to control the RATE of the MuRF I mentioned above. In this case the louder you play (louder the initial attack of a played not such as a string pluck on a bass), the faster the steps will advance on the MuRF until it just about stops advancing. The RATE knob on the MuRF will act as an offset. You can also attenuate the voltage on a CP-251 or a VX-351.
e. As Nth OSC, use Frequency Control Voltage from a keyboard into the FREQ input, then OSC to another channel in your mixing board or preferably through a filter. You’ll be amazed at how a untamed FreqBox compares to using it as an additional Oscillator meaning that it sounds as good as one built into your other Moog products.
Then my 3rd [sort of] Fooger was the VX351 which greatly enhances the possibilities and has some of the CP-251 features such as a two MULTs, two Attenuators and LFO outputs (but driven from your Voyager).
Next for me is the MP-201 which I bought myself for my wife to give me for Christmas [aren’t I a great husband] and to finish this up for now, I’ll likely be getting a MF-101 next.
While all of the options available while using keyboard based Envelopes is handy, I would love to have a filter with a different envelope running concurrent with voices created on the Voyager (Old School in my case) and also like the simple approach of patching from my rack of equipment and leaving my keyboards unmolested to some extent. It help increase the possibilities.
I was saving to purchase the Echo [it’s big bucks] but has instead opted to do something very much non-Moogy which is to purchase the Seymour Duncan Deja Vu pedal but that’s another long story.
You can’t go wrong with any of this, just read as much as you can and go to town. I bought my first Moog product off of eBay February of this year [the LP tribute]. At this point, I’m out of control.
Sorry for the long response, hopefully this info is useful for those reading it.
-Mike W. from NJ