Thanks for the Forssell link, it's a helpful description of the challenges of both summing bus designs. The Tridents I referred to are all "virtual earth" designs. The article reinforces what I've found to be the most important upgrade on these boards, adding individual ground leads from each module to a new bus bar, to improve CMRR thereby lowering noise. Here's a good write up of the procedure by Eddie Cilletti/Dan Kennedy:

http://www.digitalprosound.com/2001/...log_maint1.htm

Another option you might consider is a stand-alone 4x1 passive mixer. I use an Uptown Technologies 4x1 stereo unit that has mute and volume attenuaters on each stereo input. There are no active circuits in it so the insertion loss is 10dB, but a 1272 on it's output would make that up easy... HTH