bridging a Adcom GFA 545

Chuck - your speaker is still 8 ohms but voltage doubles making it 4x instead of 2x. If you think in terms of virtual ground that divides speaker into two 4 ohm pieces then you'll get 2x in each (same voltage, half impedance) but together it's 4x. Power supplies are not overdesigned 100% but you'll be able to handle peaks a little better. Damping factor will be twice worse (lower). It also might go over maximum current amp was designed to produce (minimum load).

I don't know if your amp can be bridged - some cannot be at all (already bridged) and others will require phase inverter for one of the amps.

If you are going to build a sub, you can always get a DVC driver.....Dual Voice Coil.....and run 1 channel to each coil.
Below about 80hz, it's all mono, anyway.
Make sure it is 4ohms or better to EACH coil.

My woofer is a JL 10W7. Its a single coil @ 3* I am very upset that it cant be bridged. The speaker is rated at 750 watts. I am running the speaker on 1 of the 2 channels. It sounds OK and this little amp is not even breaking a sweat. I need more power!!! I have a THX Pioneer Elite and in the speaker setup menu I had to add 6 db to the sub. At 3* with only 1 channel driven, how many watts would you say I am getting to the speaker? And is it ok to just run 1 channel? Thanks

A radical solution would be to SELL the Adcom and use the funds to purchase a good plate amp from maybe....Parts Express or Madisound...(Do they sell such amps?) so you can either use speaker level or line level input and get enough juice to do the driver justice.
I have an ancient Carver Cube that I shelved for similar reasons, though the cube IS bridgeable.
Adding 3db to the sub roughly doubled (2x) the power required.
HT setup is beyond me......I know there are many options. Run the front speakers as 'large'=full range or not and so on.

If that is the amp I think, the 15 yr+ old Pass design, while sturdy, you are coming up against the age of some components...mainly caps, which will eventually begin to tell.