why not just use a midi controller for it?

no worries, you’re just looking to have something that’ll set up the tempo in a more graduated fashion (or at least, with a visible BPM readout), rather than in semitones/slider position, yeah?

Any of the Korg Electribes would (and, in addition, have tap tempo), as would the Boss DR-202, The Korg KP2 & KP3 Kaoss Pads, and many mid-level synthesizers like the Roland Juno-Di. The cheapest option of these will probably be the Korg ER1 electribe (drums) or EA1 electribe (synth) at around $80-120 on ebay.

To hook them up, simply attach a midi cable from the output of the device you want to use the clock of to the midi in of the 2880, power both up, and press the ‘Ext Clock’ and ‘Quantize’ buttons on the 2880. You won’t need any audio signal from the master clock source being output, although most of the options I named above have some pretty nifty sounds and features (for instance, the KP2/3 can be used as effects boxes or samplers as well as clock sources, while the Korg Electribes can make some nice textures that’ll sync up with the audio).

it is indeed possible. Actually, you wouldn’t even need the ‘quantize’ function on to do that, then. Basically whichever device you were using for a master clock, you’d just lower the tempo ’til it you’re where you want to be on the 2880. I mean, there’ll still be steps, but rather than the 1/12th steps, you’d have steps based on a ratio of your initial tempo to your variant tempo.

I uh, actually made a simple excel spreadsheet for this, if you want a copy. just shoot me a PM with an email address of where to send this.