Just to be *crystal* clear...

0. You have a four-channel amp--we'll call those channels A, B, C, and
D.

1. Bridge A + B together to make channel "AB".
   Bridge C + D together to make channel "CD".
   You now effectively have a "two-channel" amp.

2. Wire the VC on one driver in series per the diagram to which I
pointed you.
   Do the same with the other driver.
   You now effectively have two 8 Ohm drivers.

3. Take one 8 Ohm driver and hook it up to channel AB.
   Take the other driver and hook it up to channel CD.
   You now have a two-channel amp powering two 8 Ohm speakers.

4. Relax.

-dan

It will cut your power output in half. Seeing the wattage that you have listed
for this amp is in "Peak Power" you should get around 50 - 60 watts RMS per
coil cranked up. As for your SQ, I have always found that under powering a sub
sounds like a wet fart and I have also found this to be even more true when
using lite duty amps such as Crunch. My advice is to try it out for yourself
and see how you like it.

Bob, take a deep breath. Use the force. Go to your happy place. Relax.

Cross each bridge as you come to them--don't try to get it perfect the
first time around because, well, quite frankly it sounds like this
might be your first "go" at system building and it might not sound
perfect the first time around. Even systems setup by experts typically
need tweaking and tuning to get them to sound just right...and
sometimes even that doesn't work.

Just have fun with it. Follow carefully the instructions I've given you
thus far, follow the wiring diagrams, and check everything you do two
or three times. Modern amplifiers have lots of protection circuits in
them to keep you from doing something obcenely stupid and destroying
your amplifier. That's not to say you can be careless, only that you
don't have to sweat bullets here--it's all just a matter of "putting
tab A into slot B", tightening some screws and vacuuming-up after
yourself when you're done.

Running in 8 Ohms mono is effectively the same as running in 4 Ohms
stereo. While you won't get the absolute most power out of your
amplifier, you will definitely run cooler, you'll usually have greater
dynamic headroom and the damping factor will be higher. In short, sound
quality can actually be *better* with a higher load than with a lower
load. If you find that the subs aren't quite loud enough to keep up
with the rest of your system (or with your expectations), THEN you can
fall back to using just half of your amp bridged on a 4 Ohm
load...which will be louder but *might* not sound as good--it's all
very subjective and ultimately only you are going to be able to tell if
it "sounds good" or not. We're just a bunch of talking heads and it's
not our ears in your car.

-dan

Three-channel just means one pair of channels runs bridged, the other pair
doesn't, so you'd get 500W @ 4 ohms for the bridged pair (as per your listed specs:
175W x 4-Ch @ 4-Ohms or
250W x 4-Ch @ 2-Ohms or
500W x 2-Ch @ 4-Ohms Dynamic Music Watts

Specs-wise, wiring the subs that way (500W output) wouldn't be much different
than running them as two ohm, each on its own channel (at 250W each).  There MAY
be some performance differences depending on whether the amp handles two-ohm
loads or bridged loads better, but probably nothing significant.