Biwiring: can you really hear the difference?

Depends on the system, sometimes yes, sometimes no.

Let's be more precise: Biwiring increases per/unit capacitance of the cable. If your amp can take high capacitance loads than it will improve especially bass. It's great to play with two different sets of speaker wires/cables rather than selecting one brand bi-wired.

I have used jumpers and I have bi-wired. The bi-wiring offered a much improved musical presentation. For the relatively small investment it's worth the effort. Try it!

Yes, it can make a very substantial difference.

OK... so let's say I bought two pairs of cables (4 total); my amp still only has one pair of +/- for each speaker channel. That would mean I'd have each post on my speaker accomodating two spades? Is that what people suggest?