There are only two ways which the volume level of an entire rank can be adjusted, AFAIK.
1. The way which you say you have tried, i.e. in the preset of the soundbank. Putting a positive number in the preset layer under the preset layer description will make that layer quieter the higher the number, and vice versa, and in Viena only, a negative value will increase the volume if there is no value set. There is a "maximum" of -30.
2. In the disposition, go into "construct organ", click on the stop whose volume you want to alter. Look at the "References" panel at the LH side and make sure the icon "references TO elements" is highlighted. Click on it if it isn't. Double click, on my system, on what should be the same description as the stop label. First making sure you have the "messages" panel visible along the bottom. Click on "View" at the top of the screen and then "messages" if it isn't. You should now see a number of MPL messages, starting "disengaged", "engaged", "divorced", no sorry, only joking. & etc. Look for a line that reads, "note played - set 144, set pitch, set 100". Although this last figure could technically be anything between 0 and 127. Usually its somewhere about 80-100. A higher number than set, up a maximum of 127 will increase the volume. To alter the number, place your cursor behind this number, delete the number (back space) and enter a higher number and press "Enter".
If using both above methods by inputting -30 in the soundfont preset layer and increasing the engaged level in the MPL to 127 doesn't make the rank loud enough then its a case of quietening all the other ranks instead.
The "gain value" only alters the overall volume level of the disposition.