bridging a Adcom GFA 545

The Adcom GFA-545 can't be bridged internally, it's not setup for mono operation. There used to be, many years ago, some type of device box that would bridge a stereo amp, but I haven't seen one for quite awhile. Not sure how well it worked...

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.