LFOmo

[Sacred Synthesis]

https://soundgas.com/product/soundgas-lfomo/

[Soundquest]

Another option (btw, also a UK company) I use a Kenton MK3 for adding another lfo, that can be routed for filter modulation.  Similar concept to this here, but with LED menu. The knobs offered here might be a better immediacy.   I learned that you also need a drum machine output to sync lfo though.  Clock out of the  OB6 (and other synths) don't seem to work.  So I use the Drum Machine clock to drive the OB6 and Kenton clocks.

[chysn]

It sort of reminds me of the old Moog CP-251, just add an LFO (and various other utilities) to whatever. I liked it at the time in conjunction with a Little Phatty with CV mod, but an outboard box for an LFO just kind of seems weird.

[Soundquest]

chysn, true enough, outboard is not really that practical, especially when you can't even sync it to the parent synth w/o a drum mac.  Oddly I don't crave using the additional lfo device on the OB6 as much as I thought I would.  Another thing I tried was timing the onboard phaser effect can be "sort of" a substitute, freeing up the actual lfo for something other than filter.