I don't know, but they just don't sound very aesthetically pleasing.
They just sound too "technical" and "pro-gear".
Hardly too technical. A trimmer is a potentiometer, or variable resistor. It is a passive component and has a control function only. It adds no distortion or anything deleterious.
You need ganged ones, (two on one shaft) for the surrounds,mains and center back surrounds and a single for the center. What ever is your quietest set of speakers does not have to have one. If the center is quietest you need a dual for the mains.
The preamp output goes to the outer resistor, 5 to 10 K ohm is about right. If only fine tuning is required then choose linear taper, if there are large discrepancies in channel volume then a log taper works best. The output from the sweeper goes to the input of the power amp. Grounds are common.
You use the quietest set of speakers as the reference, and then trim the pots to match with your spl. meter. I use a condenser studio microphone and mixer.
You have to buy a case for the pots. You have to drill holes to mount the pots on the front. On the back you mount female RCA sockets, one for each channel output and input. The you wire the preamp outs to the outer connectors with screened lead and wire the amp inputs to the sweeper, keeping the grounds common.
Here is a beginners guide to potentiometers.
http://sound.westhost.com/pots.htm
Here is a link to a potentiometer manufacturer. You can see the different types of controls.
http://www.ctscorp.com/components/potentiometers.htm
For this application you would need pots from this series.
http://www.ctscorp.com/components/Datasheets/270.pdf
This is a far better solution in every way to multiple preamps.
This is strange situation that has developed, as this would have been second nature to every audio enthusiast prior to 1970. Unfortunately very basic skills have become lost in this high tech age, which is a paradox.
