For those who crave a mono button in Cubase 6, but are not yet using the control room, I believe there are (at least) two ways to gitter done. Each way is similar in that it is a Cubase effect plug-in (described in the Plug-in_Reference .pdf), and it is inserted into the master fader, in slot 7 (one of the two post-fader inserts, assuming you want to keep slot eight (the other post-fader insert) free for a dithering plug-in).
As described by vic_france, the elegant way to do this is to have the plug-in bypassed during normal “stereo” listening, then when you want to listen to mono, clicking OFF the “bypass” button (i.e., letting the plug-in work) is the equivalent of a mono button.
Mix6To2 plug-in: This method of building a mono button has been around since SX3, it’s on page 47 of the plug-in reference doc. As described by vic_france here https://www.steinberg.net/forum/viewtopic.php?f=20&t=5006#p32516 , just set the two “L” faders to -6 dB. and also the two “R” faders to -6 dB. Keep the two “Output” faders up to zero, put it in the master fader slot 7, and there you are! (In my Cubase, I don’t have the 3 pairs of faders - LFe, Ls, Rs - displayed as they appear on page 47, so I don’t really know where to put those … probably down to 0 dB?).
Stereo Enhancer plug-in: As easy as the Mix6To2 method of building a mono button is … this way is even easier. The plug-in itself has a mono button! I just hit the mono button, I don’t think it matters what the other knobs are turned once that is done. Put it in that 7th insert slot in the master fader (instead of the Mix6To2 plug-in) in bypass mode, and any time you want to hear mono, just click the bypass button off.
FWIW, I tested the two plug-ins set up as mono buttons against each other, and they nulled, so I guess both work equally as well. I hadn’t heard or read a thing about using Stereo Enhancer as a mono button, can’t figure out why since it’s so easy … ?
If this needs to be revised, please do.