Since I like old 20's jazz I do not have any problem finding mono recordings. However, as you point out, if you could direct the right or left channel from the Little Dot so that you can hear the same sound from both head phones, it would be easier to compare. I am sure that this can be done, but I have abandoned this idea for the following reasons:
Truth is that I have resorted to changing the sets of tubes, shutting off the Little Dot first, and waiting a while in between changes. Amazingly enough there is a little bit of a sound stage even with old mono recordings. Certainly, in comparing how different tubes sound with good stereo recordings, I have come to the realization that the width and depth of the sound stage and the overall balance of how the frequencies are reproduced are also very important in enjoying the music. (Sometimes you hit the jackpot with a really good recording and you can hear the height as well!)
In conclusion, the timbre and the tonality of a tube is not enough to really decide what you like. You also need the overall balance of low, mid and upper frequencies and the sound stage width and depth to make a decision, and for this you need the pair of tubes. In addition, I think that the sound stage is limited with a number of headphones, so you may need to play through your speakers as well.
I am in the process of going through some 6-7 sets of tubes that I have and trying to note the differences between them. Every tube sounds a little different; some are stronger in the bass, some have a wider sound stage, some have better highs. Some tubes space out the instruments/musicians better than others for superior pinpoint imaging. Everybody has a different taste, but I think that you could reach a consensus which tubes are the best sounding overall.
However, the MarkIII is such a nice piece of equipment that almost anything sounds good with it!