Ok people, i've finally found out how to sample audio-in source (guitar, bass and so on) with sp-606 metronome!

go to system settings, press metronome - "always"

TA DA! and now your metronome plays whith sp-606 on!

press "sampling" - and... wow! - you hear metronome playin while you're sampling!

now it's much easier to sample bass or guitar riffs into sp-606

I assume it doesn't actually record the sound of the metronome?

That's REALLY useful; good tip!

Yeah, it doesn't record the metronome.

excellent, especially if you only have one machine to work with. I've always used a ride from another drum machine in my headphones to play along to. No need though with this tip

That's golden, on my 303 I just had to get fairly good at doing it by listening to the rhythm in the sample source.

That's a really good method too, as it influences your playing with the groove of the sample.