Just saw this thread and had to chime in as that is exactly what I had a friend of mine build: 24ch passive summer with four channels that are switchable like the folcrum. We used DB 25 connectors and put it inside the volume/monitor box he made for me some years back. I use the SSL AlphaLink so the combo works real nice. The mono channels are up a few DB which helps on the LV/kick/bass channels.
I've just been doing some mixing through it over the past few weeks, from English Folk to Neo-goth metal, and have loved the results. I am making up gain with a Sytek using the Burr Brown opamps. Nothing like bringing a little amp clipping back into the mix!!! METAL.
Something about the punch I'm now getting that does not happen ITB. Also, imaging is.......
different. I like it. I could go on but it would be rambling... Suffice to say, I can accurately A/B the ITB and Summer mixes, level matched, and notice subtle musical things that are different, hard to describe well, but..... I like it! Have yet to try the other pre's lying around API, Avalon.... hmmmmmm
-bassman