JB, On the subject of changing capacitors, the basis for choosing the value of the output coupling capacitors is to provide for the best possible extreme low frequency response, first of all. (f = 1/2*pi*R*C, where R is the net impedance seen by the capacitor in Ohms, and C is the capacitance in Farads). That calculation determines the minimum value of the coupling capacitor, depending upon where you want to cut-off the LF response. The next consideration is the overall SQ of the capacitor that you choose. In this case, you say you switched from the OEM 30uF capacitor to 3.3uF and then to 12uF. 30uF is going to be way overkill using any imaginable amplifier, in achieving a high pass filter that goes down below ~2Hz, which is more than adequate. But lower values of capacitance can also give you a 2Hz cut-off frequency or near enough to it in the real world. That depends upon the parallel sum of the impedances that the capacitor works into. Like I said above, nearly any amplifier you might own (solid state or tube) will have at least a 10K ohm input impedance. Most will be higher. 10uF capacitance is sufficient yield a ~2 Hz high pass filter into 10K ohms. (Insert the values into the equation above, to see what I mean.) If you are driving a tube amplifier with a more typical 100K ohm input impedance, then even a 1uF capacitor will do the job. The next question is SQ. Yes, it is possible that your 12uF capacitor just happens to sound better than your 3.3uF capacitor; you haven't stated the types or brands that you chose. In that case, the 12uF will sound better simply for the fact it IS better, not because a higher value is necessarily doing you any particular good for low bass response. In the case of your Chinook and my Steelhead, the OEM coupling capacitors are 30uF in value and the brand is either REL or MIT (made by REL) metallized film type. REL make some wonderful capacitors, but this particular type is not among them, in my opinion. So I made my changes not only because there is no point in using such a large value capacitor in this application, but in addition because the particular 30uF capacitor they chose is not optimal with respect to SQ, in my own opinion.
Manley were concerned with cable capacitance, when they chose to use such a high value of coupling capacitor and to insert the 47 ohm resistor in series with the output. Their choice enables the units to drive very long runs of cable with high capacitance, without messing up the frequency response, but I just viewed their choice in that case as overkill. I would never use such long and capacitative cables in the first place. Many manufacturers make choices like this simply because they count on the end user abusing their product, e.g., by asking the preamp to drive 40 feet of high capacitance cable. Then the end user comes away critical of the product when it fails to perform well under such a ridiculous stress.
