None of this is advised or recommended, as for bi-wiring and bi-amping, it is generally accepted that there is no audible benefit. It sounds cool, costs money to do, generally won't harm anything... but yields next to nothing.
If your speakers have two sets of binding posts, and many do, these days, they connect to the crossover (XO) inside the speaker, and there is a jumper between the two positive and between the two negative posts.
After removing those jumpers, one could:
Bi-wire is connecting two sets of speaker cables to the amplifer binding posts, and then connecting one set to the LF, and one set to the HF binding posts of the speaker. Most notably, all this really does is decrease the resistance that the amplifier sees (IIRC).
Passive Bi-amp is more complicated, and I will only briefly describe it. You would need TWO amplifiers, and two sets of cables. One amplifer would be connected to the LF binding posts (with jumpers removed) and the other amplifer would be connected to the HF binding posts. You would split the signal from the AVR pre out, one two each amplifier. There are various ways to achieve this, but in the end, the result is still largely inaudible. There is another method that is much more advanced and expensive called Active Bi-Amping.
Lastly, you asked about Bridging. Stereo amplifier sometimes come wit ha feature called bridged mode, which turns the 2-channel amplifier into a Mono amplifier. It will usually double the wattage being sent out, and IIRC, halve the impedance. This also is a more advanced technique, because if you don't do it properly, you can fry your speakers and your amp.
Again, You really have no reason to consider doing any of this. Connect your system up simply and correctly. The Denon has enough power to handle your Ultra towers in two channel mode. When you get to 5.x.2 or .4 atmos, then having an amp for your front three is much more useful.
Some APC units are good, Trip Lite makes good ones, Monster is OK... look for one that is rated for 20Amps, and has a high rating for the joules it can protect your gear from. A lot of them come with 'insurance' saying that they will replace up $50,000 of gear if their product fails. Shouldn't cost much more than $50-$100 depending on the brand.
For our friend here... starting off simple is best. Though I would agree that your center channel and subs should take priority over amplifiers. That's just my way of looking at it though.
