My good old Mac MC2105 has both left and right gain control (I think many power amp had that back in the '60s) and my question is what is the "optimal" combination of Preamp vol setting with the Power Amp gain setting or if there's any rule of thumb here. I saw some web info. suggest I should turn the power amp gain knobs all the way up (as if it doesn't have the control... I guess) which will give me the cleanest sound but I'm not sure.
Chutt, have you tried both methods? It seems to me that the amp should be up, although with a caveat. Your older amp should be very clean and proper to use the volume controls in a position which they are(presumably) not accustomed to.
The channel gain controls are best used fully open on the power amp. The stereo balance control is usually set for a center balance position. That effectively removes them from the circuit. You can use the gain controls to lower the input sensitivity if, you have a system where you do not have full range control on you pre amp's volume control or you have other amplifiers that require matching levels to each other.
In the first case, not enough range on the pre amp's vc, you might find a good example when new technology comes on the market. When CD's came along their output was substantially higher than that of the typical phono cartridge. This meant you would have a listenable volume at a lower portion of the pre amp's vc. You might even get to the point where you had a difficult time adjusting the pre amp's vc without having too much change for each detent of the control. If this was the case, you would be better off, convenience wise, to turn down the gain controls on the power amplifiers. This would give less change in volume for each detent increase from the pre amp and a widfer range of control for comfortable istening levels.
Also, pre amps work best when their volume controls are as close to fully open as possible. Since pre amps deal mainly with the very lowest level signals in a system, the pre amp is the most sensitive piece of equipment for signal loss and degradation. After the sources themself, of course. By running the pre amp's vc fully open, you take it out of the circuit and reduce the risk of the volume control affecting the sound. Ideally, then, you would adjust the power amp's gain control to give you a setting on the pre amp that was as close to fully open as possible for a comfortable listening level. This can get somewhat dicey since the two channel gain controls on the power amp don't always track identically to one another. If you are using the stereo gain control on the Mac amp, you can get better results by using both controls or you can use the stereo gain control to adjust for a system imbalance and not use the pre amp's balance control.
Since this involves a bit of try-and-listen to get the best balance between the various settings on the pre amd power amplifiers controls, it is usually recommended that you just set the gain controls on the amp to fully open and operate from the pre amp unless the system dictates otherwise.
Also, keep in mind Mac sold to quite a few laboratories, mainly audiologists, who required the use of gain controls and so Mac included the gain controls as a useful feateur for these clients. The controls are a holdover from the time when Mac produced mono amps only. Then the "new" technology of stereo required a second amp within the system. While all Mac amps maintained the same input sensitivity, the new second channel amp might require some tweaking of the gain control if you weren't using a Mac pre amp.