A / B Switching

btw,

It isn't good to have spare speakers in a listening space, they passively react to vibrations generated by the active speakers. 

This is good stuff - thank you to everyone.  So if I’m following correctly: Mac C 28 out to the Mac 2105.  Also out of the C28 to Adcom GFA555.  From there as normal to speakers. The C 28 preamp doesn’t have any type of selector to switch between amps (?). It does however have the RCA posts to connect another amp.  I’m a little new to vintage audio so please forgive any obvious questions 

OK let me look.

That one uses a Main and Remote push button option on the front Same as ok.. Don’t just push the button, while blaring either, I don’t know what will happen.. OK.. Turn it DOWN then push the button.. Just to be safe.

Remote and Main do not have independent volume control, (usually)
Tone control on one effect ALL the amps..

Route one set of RCAs from each, The mains and the remotes to the two different amps.. That Adcom be careful. With some of them and I’m not sure on that one..

The GFA 565s the DC voltage goes high and blows voice coils on the bass drivers..It has to do with a 6db first order XO there is NO cap in the path (speaker side). They (Adcoms) have leaky Dcaps and causes the IC boards to short.. A MESS. BUT Adcom is a great amp for the money. I still have 6-8 around here..I like the 5800s, Power house..Good sounding..when you get it warm..

It is easy to check DC offset, just google it, and see if the 555 is prone to.. Save you some money..Some bass drivers are tough to come by..

Have fun..

Regards..