More inputs were needed (+ I like redundency): 2 reel to reels, 2 dual bay cassette decks, 2 turntables (the one you see in the first picture is a Technics SL-M3, the unseen one is a DUAL 1229), 3 FM tuners, 6 NAD 2200's, 2 NAD 2100's, 1 PROTON d1200, 2 pair of Dahlquist M-905's and 2 custom built (by me) subs.
And:
Quadraphonic Synthesis
With two Holman Preamplifiers, you can synthesis and control four output channels from just two input channels.
Apply all your inputs to the first Holman Preamplifier. Use it for all your tone controls, filters and source and tape selections. Leave its Stereo Mode in Stereo.
Connect the first Holman Preamplifier's MAIN 1 output to one power amplifier and your two front speakers.
Connect the first Holman Preamplifier's MAIN 2 output to any line-level input of the second Holman Preamplifier.
Connect the second Holman Preamplifier's MAIN 1 output to the power amplifier for your two rear speakers.
Rotate the second Holman Preamplifier's stereo mode control to L-R, and start by setting it to about unity gain or a bit less, and keep its tone controls flat. The tone and filter settings of the first Holman Preamplifier are fed automatically to the second Holman Preamplifier.
Set balance on the first preamp. Set front-rear balance on the Volume control of the second preamp.
Leave the power switch of the second preamp ON, and plug its power cord into a switched outlet of the first Holman Preamplifier. Now the power is controlled by the first Holman Preamplifier, too.